Area and volume of prisms and cylinders
By the end of this chapter you will be able to
- Find the area of a trapezium and of a compound shape made from rectangles and triangles
- Find the area and circumference of a circle, and give answers in terms of pi
- Find the volume of a cuboid and of a triangular prism, using the area of the cross-section
- Find the volume and curved surface area of a cylinder, and give answers in terms of pi
- Find the surface area of a cuboid, and convert square and cubic units such as m2 to cm2
You should already know
- Area of rectangles, triangles and parallelograms, and the formula for the area of a rectangle (Year 7)
- Multiplying decimals and whole numbers, and the square of a number (Year 7 and Year 8, Chapter 9)
- Units of length, and the relationship between cm and m (Year 7)
Where this leads
- Circle theorems, sectors, arcs and surface areas of cones and spheres (Year 9 and GCSE)
- Volume and surface area of more complex solids, and the use of formulae (Year 9)
- Similar shapes and the effect of a scale factor on area and volume (Year 9)
National Curriculum for England, Key Stage 3 mathematics, Year 8. Content: Use area formulae for trapeziums, circles and compound shapes, and convert units of area, Find the volume of a cuboid, and of a prism with a uniform cross-section, as the area of the cross-section times the length, Find the volume and curved surface area of a cylinder, using pi, Find the surface area of a cuboid or a prism.
Exercise P Do you remember?
Non-calculatorMethod. Area of a rectangle = length × width.
- Write the formula. Area = length × width.8 × 3
- Multiply. Eight threes are 24.24 cm2
Check. 24 ÷ 3 = 8.
Method. Multiply the numbers in order.
- Multiply the first two. 6 × 4 = 24.24
- Multiply by 2. 24 × 2 = 48.48
Check. 48 ÷ 2 = 24.
Method. There are 100 centimetres in 1 metre.
- Recall the conversion. 1 m = 100 cm.1 m = 100 cm
- Write the answer. 100 cm
Check. 100 cm ÷ 100 = 1 m.
Method. Work out the square first, then multiply by 2.
- Square 3. 32 = 9.9
- Multiply by 2. 9 × 2 = 18.18
Check. 18 ÷ 2 = 9.
Method. A square has four equal sides.
- Add the four sides. Each side is 5 cm.5 + 5 + 5 + 5
- Write the answer. 20 cm
Check. 4 × 5 = 20.
Area of trapeziums and compound shapes
A trapezium has two parallel sides. Its area is the average of the parallel sides, multiplied by the height (the perpendicular distance between them).
A = (a + b) ÷ 2 × h, where a and b are the parallel sides.
A compound shape is made from simpler shapes. Split it into rectangles and triangles, find the area of each part, then add them.
Key ideas
- Trapezium area: (a + b) ÷ 2 × h.
- The height is perpendicular to the parallel sides, not the slanted side.
- For a compound shape, split it into parts and add the areas.
- If part of a shape is missing, subtract its area from the whole.
Method. The area of a trapezium is the average of the two parallel sides, times the height.
- Write the formula. Area = (a + b) ÷ 2 × h.A = (a + b) ÷ 2 × h
- Add the parallel sides. The parallel sides are 5 cm and 9 cm.5 + 9 = 14
- Halve and multiply by the height. Half of 14 is 7, then multiply by 4 cm.7 × 4 = 28
- Write the answer. 28 cm2
Check. The average of 5 and 9 is 7.0, and 7.0 × 4 = 28.0.
Watch out. Use the perpendicular height, not the slanted side.
Method. Find the area of each rectangle, then add them.
- Area of the first rectangle. 8 × 3 = 24 cm2.24
- Area of the second rectangle. 4 × 5 = 20 cm2.20
- Add the areas. 24 + 20 = 44.44 cm2
Check. The two rectangles do not overlap, so their areas add.
Method. Find the area of the whole rectangle, then subtract the cut-out part.
- Area of the rectangle. 10 × 6 = 60 cm2.60
- Area of the cut-out. 2 × 2 = 4 cm2.4
- Subtract. 60 − 4 = 56.56 cm2
Check. 56 + 4 = 60.
Watch out
Watch out: use the perpendicular height. A slanted side of a trapezium is longer than the height, so using it gives the wrong area.
Exam tip
Exam tip. Write the formula, then put in the numbers in order. Give the units in your final answer.
Exercise 10A Area of trapeziums and compound shapes
Non-calculatorMethod. The area of a trapezium is the average of the two parallel sides, times the height.
- Write the formula. Area = (a + b) ÷ 2 × h.A = (a + b) ÷ 2 × h
- Add the parallel sides. The parallel sides are 5 cm and 9 cm.5 + 9 = 14
- Halve and multiply by the height. Half of 14 is 7, then multiply by 4 cm.7 × 4 = 28
- Write the answer. 28 cm2
Check. The average of 5 and 9 is 7.0, and 7.0 × 4 = 28.0.
Watch out. Use the perpendicular height, not the slanted side.
Method. The area of a trapezium is the average of the two parallel sides, times the height.
- Write the formula. Area = (a + b) ÷ 2 × h.A = (a + b) ÷ 2 × h
- Add the parallel sides. The parallel sides are 6 cm and 10 cm.6 + 10 = 16
- Halve and multiply by the height. Half of 16 is 8, then multiply by 5 cm.8 × 5 = 40
- Write the answer. 40 cm2
Check. The average of 6 and 10 is 8.0, and 8.0 × 5 = 40.0.
Watch out. Use the perpendicular height, not the slanted side.
Method. The area of a trapezium is the average of the two parallel sides, times the height.
- Write the formula. Area = (a + b) ÷ 2 × h.A = (a + b) ÷ 2 × h
- Add the parallel sides. The parallel sides are 3 cm and 7 cm.3 + 7 = 10
- Halve and multiply by the height. Half of 10 is 5, then multiply by 6 cm.5 × 6 = 30
- Write the answer. 30 cm2
Check. The average of 3 and 7 is 5.0, and 5.0 × 6 = 30.0.
Watch out. Use the perpendicular height, not the slanted side.
Method. The area of a trapezium is the average of the two parallel sides, times the height.
- Write the formula. Area = (a + b) ÷ 2 × h.A = (a + b) ÷ 2 × h
- Add the parallel sides. The parallel sides are 8 cm and 12 cm.8 + 12 = 20
- Halve and multiply by the height. Half of 20 is 10, then multiply by 3 cm.10 × 3 = 30
- Write the answer. 30 cm2
Check. The average of 8 and 12 is 10.0, and 10.0 × 3 = 30.0.
Watch out. Use the perpendicular height, not the slanted side.
Method. The area of a trapezium is the average of the two parallel sides, times the height.
- Write the formula. Area = (a + b) ÷ 2 × h.A = (a + b) ÷ 2 × h
- Add the parallel sides. The parallel sides are 4 cm and 10 cm.4 + 10 = 14
- Halve and multiply by the height. Half of 14 is 7, then multiply by 7 cm.7 × 7 = 49
- Write the answer. 49 cm2
Check. The average of 4 and 10 is 7.0, and 7.0 × 7 = 49.0.
Watch out. Use the perpendicular height, not the slanted side.
Method. The area of a trapezium is the average of the two parallel sides, times the height.
- Write the formula. Area = (a + b) ÷ 2 × h.A = (a + b) ÷ 2 × h
- Add the parallel sides. The parallel sides are 2 cm and 8 cm.2 + 8 = 10
- Halve and multiply by the height. Half of 10 is 5, then multiply by 5 cm.5 × 5 = 25
- Write the answer. 25 cm2
Check. The average of 2 and 8 is 5.0, and 5.0 × 5 = 25.0.
Watch out. Use the perpendicular height, not the slanted side.
Method. The area of a trapezium is the average of the two parallel sides, times the height.
- Write the formula. Area = (a + b) ÷ 2 × h.A = (a + b) ÷ 2 × h
- Add the parallel sides. The parallel sides are 7 cm and 11 cm.7 + 11 = 18
- Halve and multiply by the height. Half of 18 is 9, then multiply by 2 cm.9 × 2 = 18
- Write the answer. 18 cm2
Check. The average of 7 and 11 is 9.0, and 9.0 × 2 = 18.0.
Watch out. Use the perpendicular height, not the slanted side.
Method. Find the area of each rectangle, then add them.
- Area of the first rectangle. 8 × 3 = 24 cm2.24
- Area of the second rectangle. 4 × 5 = 20 cm2.20
- Add the areas. 24 + 20 = 44.44 cm2
Check. The two rectangles do not overlap.
Method. Subtract the cut-out from the whole rectangle.
- Area of the rectangle. 10 × 6 = 60 cm2.60
- Area of the cut-out. 2 × 2 = 4 cm2.4
- Subtract. 60 − 4 = 56.56 cm2
Check. 56 + 4 = 60.
Method. Find the area of each rectangle, then add them.
- Area of the first rectangle. 12 × 4 = 48 cm2.48
- Area of the second rectangle. 3 × 6 = 18 cm2.18
- Add. 48 + 18 = 66.66 cm2
Check. The two rectangles are joined without overlap.
Method. The area of a trapezium is the average of the two parallel sides, times the height.
- Write the formula. Area = (a + b) ÷ 2 × h.A = (a + b) ÷ 2 × h
- Add the parallel sides. The parallel sides are 4 cm and 8 cm.4 + 8 = 12
- Halve and multiply by the height. Half of 12 is 6, then multiply by 3 cm.6 × 3 = 18
- Write the answer. 18 cm2
Check. The average of 4 and 8 is 6.0, and 6.0 × 3 = 18.0.
Watch out. Use the perpendicular height, not the slanted side.
Method. The height must be perpendicular to the parallel sides. The slanted side is not the height.
- Look at the height. The height is the perpendicular distance between the parallel sides.perpendicular height
- Compare. A slanted side is longer than the height, so the area would be too big.slanted > height
- Write the answer. Ali is not right: the slanted side must not be used.No
Check. Use the perpendicular height given in the question.
Method. Add the area of the extra part to the area of the rectangle.
- Write the sum. The total area is 45 + 2.45 + 2 = 47
- Check the units. Both areas are in cm2, so they can be added.cm2 + cm2
- Write the answer. The total is 47 cm2.47 cm2
Check. 47 − 2 = 45, which is the rectangle.
Method. Use the formula backwards: A = (a + b) ÷ 2 × h.
- Write the formula. A = (a + b) ÷ 2 × h.36 = (4 + 8) ÷ 2 × h
- Simplify. (4 + 8) ÷ 2 = 6.36 = 6h
- Solve for h. Divide both sides by 6.h = 6
- Write the answer. 6 cm
Check. (4 + 8) ÷ 2 × 6 = 36.
Area and circumference of a circle
The radius r is the distance from the centre to the edge. The diameter is twice the radius.
The area of a circle is π r2. The circumference (distance round) is 2π r.
Leave the answer in terms of π (for example 25π) when you are asked for an exact answer, so you do not need a calculator.
Key ideas
- Area of a circle = π r2.
- Circumference = 2π r = π d.
- Square the radius before you multiply by π.
- Use the radius, not the diameter, in the area formula.
Method. The area of a circle is π r2. Leave the answer in terms of π.
- Write the formula. Area = π r2.A = π r2
- Square the radius. The radius is 6 cm, so r2 = 6 × 6.62 = 36
- Multiply by π. Leave π in the answer.A = 36π
- Write the answer. 36π cm2
Check. If you use π = 3.14, the area is about 113 cm2.
Watch out. Square the radius first, not the diameter. The diameter is twice the radius.
Method. The circumference of a circle is 2π r. Leave the answer in terms of π.
- Write the formula. Circumference = 2π r.C = 2π r
- Put in r. The radius is 4 cm.2 × π × 4
- Multiply. Multiply the numbers, and keep π.C = 8π
- Write the answer. 8π cm
Check. The circumference is about 3 times the diameter (8 cm), so about 24 cm.
Watch out. Use the radius with the formula 2πr, or the diameter with πd.
Method. A semicircle is half a circle, so its area is half the area of the full circle.
- Find the area of the full circle. Area = π × 62 = 36π.36π
- Halve it. A semicircle is half the circle.36π ÷ 2 = 18π
- Write the answer. 18π cm2
Check. 18π is half of 36π.
Watch out
Watch out: the area is π r2, not 2π r. Square the radius first, and do not use the diameter in the area formula.
Exam tip
Exam tip. If a question says “give your answer in terms of π”, leave the π in the answer. Do not write a decimal.
Exercise 10B Area and circumference of circles
Non-calculatorMethod. The area of a circle is π r2. Leave the answer in terms of π.
- Write the formula. Area = π r2.A = π r2
- Square the radius. The radius is 3 cm, so r2 = 3 × 3.32 = 9
- Multiply by π. Leave π in the answer.A = 9π
- Write the answer. 9π cm2
Check. If you use π = 3.14, the area is about 28 cm2.
Watch out. Square the radius first, not the diameter. The diameter is twice the radius.
Method. The area of a circle is π r2. Leave the answer in terms of π.
- Write the formula. Area = π r2.A = π r2
- Square the radius. The radius is 5 cm, so r2 = 5 × 5.52 = 25
- Multiply by π. Leave π in the answer.A = 25π
- Write the answer. 25π cm2
Check. If you use π = 3.14, the area is about 78 cm2.
Watch out. Square the radius first, not the diameter. The diameter is twice the radius.
Method. The area of a circle is π r2. Leave the answer in terms of π.
- Write the formula. Area = π r2.A = π r2
- Square the radius. The radius is 6 cm, so r2 = 6 × 6.62 = 36
- Multiply by π. Leave π in the answer.A = 36π
- Write the answer. 36π cm2
Check. If you use π = 3.14, the area is about 113 cm2.
Watch out. Square the radius first, not the diameter. The diameter is twice the radius.
Method. The area of a circle is π r2. Leave the answer in terms of π.
- Write the formula. Area = π r2.A = π r2
- Square the radius. The radius is 10 cm, so r2 = 10 × 10.102 = 100
- Multiply by π. Leave π in the answer.A = 100π
- Write the answer. 100π cm2
Check. If you use π = 3.14, the area is about 314 cm2.
Watch out. Square the radius first, not the diameter. The diameter is twice the radius.
Method. The area of a circle is π r2. Leave the answer in terms of π.
- Write the formula. Area = π r2.A = π r2
- Square the radius. The radius is 4 cm, so r2 = 4 × 4.42 = 16
- Multiply by π. Leave π in the answer.A = 16π
- Write the answer. 16π cm2
Check. If you use π = 3.14, the area is about 50 cm2.
Watch out. Square the radius first, not the diameter. The diameter is twice the radius.
Method. The area of a circle is π r2. Leave the answer in terms of π.
- Write the formula. Area = π r2.A = π r2
- Square the radius. The radius is 7 cm, so r2 = 7 × 7.72 = 49
- Multiply by π. Leave π in the answer.A = 49π
- Write the answer. 49π cm2
Check. If you use π = 3.14, the area is about 154 cm2.
Watch out. Square the radius first, not the diameter. The diameter is twice the radius.
Method. The circumference of a circle is 2π r. Leave the answer in terms of π.
- Write the formula. Circumference = 2π r.C = 2π r
- Put in r. The radius is 4 cm.2 × π × 4
- Multiply. Multiply the numbers, and keep π.C = 8π
- Write the answer. 8π cm
Check. The circumference is about 3 times the diameter (8 cm), so about 24 cm.
Watch out. Use the radius with the formula 2πr, or the diameter with πd.
Method. The circumference of a circle is 2π r. Leave the answer in terms of π.
- Write the formula. Circumference = 2π r.C = 2π r
- Put in r. The radius is 7 cm.2 × π × 7
- Multiply. Multiply the numbers, and keep π.C = 14π
- Write the answer. 14π cm
Check. The circumference is about 3 times the diameter (14 cm), so about 42 cm.
Watch out. Use the radius with the formula 2πr, or the diameter with πd.
Method. The circumference of a circle is 2π r. Leave the answer in terms of π.
- Write the formula. Circumference = 2π r.C = 2π r
- Put in r. The radius is 5 cm.2 × π × 5
- Multiply. Multiply the numbers, and keep π.C = 10π
- Write the answer. 10π cm
Check. The circumference is about 3 times the diameter (10 cm), so about 30 cm.
Watch out. Use the radius with the formula 2πr, or the diameter with πd.
Method. A semicircle is half a circle.
- Find the area of the full circle. Area = π × 62 = 36π.36π
- Halve it. A semicircle is half the circle.36π ÷ 2 = 18π
- Write the answer. 18π cm2
Check. 18π is half of 36π.
Method. The circumference of a circle is 2π r. Leave the answer in terms of π.
- Write the formula. Circumference = 2π r.C = 2π r
- Put in r. The radius is 5 cm.2 × π × 5
- Multiply. Multiply the numbers, and keep π.C = 10π
- Write the answer. 10π cm
Check. The circumference is about 3 times the diameter (10 cm), so about 30 cm.
Watch out. Use the radius with the formula 2πr, or the diameter with πd.
Method. Find the radius from the diameter, then use the area formula.
- Find the radius. The radius is half the diameter: 8 ÷ 2 = 4 cm.r = 4
- Find the area. Area = π × 42 = 16π.16π
- Write the answer. 16π cm2
Check. The diameter is 8, so the radius is 4.
Method. Square the radius, then multiply by π.
- Square the radius. 32 = 9.9
- Multiply by π. Area = 9π, not 6π.9π ≠ 6π
- Write the answer. Ali is not right. The area is 9π cm2.No
Check. 6π is the circumference of a circle of radius 3 cm, not the area.
Method. Work out both, then explain the difference.
- Area. Area = π × 52 = 25π cm2.25π
- Circumference. Circumference = 2π × 5 = 10π cm.10π
- Explain. The area is the space inside (square units). The circumference is the distance round (units).25π cm2 and 10π cm
Check. Area uses r2, so its units are squared.
Method. The area of a circle is π r2. Leave the answer in terms of π.
- Write the formula. Area = π r2.A = π r2
- Square the radius. The radius is 10 cm, so r2 = 10 × 10.102 = 100
- Multiply by π. Leave π in the answer.A = 100π
- Write the answer. 100π cm2
Check. If you use π = 3.14, the area is about 314 cm2.
Watch out. Square the radius first, not the diameter. The diameter is twice the radius.
Volume of cuboids and prisms
The volume of a cuboid is length × width × height.
A prism has the same cross-section all the way along its length. Its volume is area of the cross-section × length.
For a triangular prism, the cross-section is a triangle, with area ½ × base × height.
Key ideas
- Cuboid volume = length × width × height.
- Prism volume = area of the cross-section × length.
- Volume is measured in cubic units, such as cm3.
- For a triangular cross-section, the area is ½ × base × height.
Method. The volume of a cuboid is length × width × height.
- Write the formula. Volume = length × width × height.V = l × w × h
- Put in the numbers. The sides are 5 cm, 4 cm and 3 cm.5 × 4 × 3
- Multiply. Multiply the three numbers.20 × 3 = 60
- Write the answer. 60 cm3
Check. Check: 60 ÷ 3 = 20, which is the area of the base.
Watch out. Write the answer in cubic units, cm3.
Method. For a prism, the volume is the area of the cross-section × the length.
- Find the area of the cross-section. This is the area of the end face of the prism.area = 12 cm2
- Multiply by the length. The prism is 10 cm long.12 × 10 = 120
- Write the answer. 120 cm3
Check. Check: 120 ÷ 10 = 12, the cross-section area.
Watch out. Use the area of the cross-section (the end), not the area of the long face.
Method. First find the area of the triangular cross-section, then multiply by the length.
- Find the area of the cross-section. Area of a triangle = ½ × base × height.½ × 6 × 4 = 12 cm2
- Multiply by the length. The prism is 10 cm long.12 × 10 = 120
- Write the answer. 120 cm3
Check. 12 ÷ 10 would give the cross-section again.
Watch out
Watch out: the volume is in cubic units, cm3. Do not give a volume with squared units.
Exam tip
Exam tip. For a prism, write the area of the cross-section first. Then multiply by the length. Check the units.
Exercise 10C Volume of cuboids and prisms
Non-calculatorMethod. The volume of a cuboid is length × width × height.
- Write the formula. Volume = length × width × height.V = l × w × h
- Put in the numbers. The sides are 5 cm, 4 cm and 3 cm.5 × 4 × 3
- Multiply. Multiply the three numbers.20 × 3 = 60
- Write the answer. 60 cm3
Check. Check: 60 ÷ 3 = 20, which is the area of the base.
Watch out. Write the answer in cubic units, cm3.
Method. The volume of a cuboid is length × width × height.
- Write the formula. Volume = length × width × height.V = l × w × h
- Put in the numbers. The sides are 8 cm, 2 cm and 7 cm.8 × 2 × 7
- Multiply. Multiply the three numbers.16 × 7 = 112
- Write the answer. 112 cm3
Check. Check: 112 ÷ 7 = 16, which is the area of the base.
Watch out. Write the answer in cubic units, cm3.
Method. The volume of a cuboid is length × width × height.
- Write the formula. Volume = length × width × height.V = l × w × h
- Put in the numbers. The sides are 10 cm, 10 cm and 10 cm.10 × 10 × 10
- Multiply. Multiply the three numbers.100 × 10 = 1000
- Write the answer. 1000 cm3
Check. Check: 1000 ÷ 10 = 100, which is the area of the base.
Watch out. Write the answer in cubic units, cm3.
Method. The volume of a cuboid is length × width × height.
- Write the formula. Volume = length × width × height.V = l × w × h
- Put in the numbers. The sides are 6 cm, 5 cm and 2 cm.6 × 5 × 2
- Multiply. Multiply the three numbers.30 × 2 = 60
- Write the answer. 60 cm3
Check. Check: 60 ÷ 2 = 30, which is the area of the base.
Watch out. Write the answer in cubic units, cm3.
Method. The volume of a cuboid is length × width × height.
- Write the formula. Volume = length × width × height.V = l × w × h
- Put in the numbers. The sides are 3 cm, 3 cm and 9 cm.3 × 3 × 9
- Multiply. Multiply the three numbers.9 × 9 = 81
- Write the answer. 81 cm3
Check. Check: 81 ÷ 9 = 9, which is the area of the base.
Watch out. Write the answer in cubic units, cm3.
Method. The volume of a cuboid is length × width × height.
- Write the formula. Volume = length × width × height.V = l × w × h
- Put in the numbers. The sides are 7 cm, 4 cm and 4 cm.7 × 4 × 4
- Multiply. Multiply the three numbers.28 × 4 = 112
- Write the answer. 112 cm3
Check. Check: 112 ÷ 4 = 28, which is the area of the base.
Watch out. Write the answer in cubic units, cm3.
Method. For a prism, the volume is the area of the cross-section × the length.
- Find the area of the cross-section. This is the area of the end face of the prism.area = 12 cm2
- Multiply by the length. The prism is 10 cm long.12 × 10 = 120
- Write the answer. 120 cm3
Check. Check: 120 ÷ 10 = 12, the cross-section area.
Watch out. Use the area of the cross-section (the end), not the area of the long face.
Method. For a prism, the volume is the area of the cross-section × the length.
- Find the area of the cross-section. This is the area of the end face of the prism.area = 20 cm2
- Multiply by the length. The prism is 5 cm long.20 × 5 = 100
- Write the answer. 100 cm3
Check. Check: 100 ÷ 5 = 20, the cross-section area.
Watch out. Use the area of the cross-section (the end), not the area of the long face.
Method. For a prism, the volume is the area of the cross-section × the length.
- Find the area of the cross-section. This is the area of the end face of the prism.area = 9 cm2
- Multiply by the length. The prism is 7 cm long.9 × 7 = 63
- Write the answer. 63 cm3
Check. Check: 63 ÷ 7 = 9, the cross-section area.
Watch out. Use the area of the cross-section (the end), not the area of the long face.
Method. For a prism, the volume is the area of the cross-section × the length.
- Find the area of the cross-section. This is the area of the end face of the prism.area = 15 cm2
- Multiply by the length. The prism is 4 cm long.15 × 4 = 60
- Write the answer. 60 cm3
Check. Check: 60 ÷ 4 = 15, the cross-section area.
Watch out. Use the area of the cross-section (the end), not the area of the long face.
Method. First find the area of the triangular cross-section, then multiply by the length.
- Find the area of the cross-section. ½ × 6 × 4 = 12 cm2.12
- Multiply by the length. 12 × 10 = 120.120
- Write the answer. 120 cm3
Check. 120 ÷ 10 = 12.
Method. The volume of a cuboid is length × width × height.
- Write the formula. Volume = length × width × height.V = l × w × h
- Put in the numbers. The sides are 20 cm, 15 cm and 8 cm.20 × 15 × 8
- Multiply. Multiply the three numbers.300 × 8 = 2400
- Write the answer. 2400 cm3
Check. Check: 2400 ÷ 8 = 300, which is the area of the base.
Watch out. Write the answer in cubic units, cm3.
Method. Multiply the three lengths.
- Multiply. 2 × 3 × 4 = 24.24
- Compare. Ali said 9, which is 2 + 3 + 4 = 9, adding instead of multiplying.9 ≠ 24
- Write the answer. Ali is not right. The volume is 24 cm3.No
Check. Volume is found by multiplying the three lengths.
Method. Multiply the cross-section area by the length.
- Multiply. 0.5 × 4 = 2.2
- Write the answer. 2 m3
Check. 2 ÷ 4 = 0.5.
Method. Use the volume formula backwards: height = volume ÷ (length × width).
- Find the area of the base. 5 × 4 = 20 cm2.20
- Divide. Height = 60 ÷ 20.60 ÷ 20 = 3
- Write the answer. 3 cm
Check. 20 × 3 = 60.
Method. For a prism, the volume is the area of the cross-section × the length.
- Find the area of the cross-section. This is the area of the end face of the prism.area = 7 cm2
- Multiply by the length. The prism is 12 cm long.7 × 12 = 84
- Write the answer. 84 cm3
Check. Check: 84 ÷ 12 = 7, the cross-section area.
Watch out. Use the area of the cross-section (the end), not the area of the long face.
Volume and curved surface area of a cylinder
A cylinder is a prism with a circular cross-section.
Its volume is the area of the circle times the height: π r2 h.
Its curved surface area is the curved side unrolled into a rectangle: 2π r h.
Key ideas
- Volume of a cylinder = π r2 h.
- Curved surface area = 2π r h.
- Give the answer in terms of π if you are not using a calculator.
- Square the radius first, then multiply by the height.
Method. The volume of a cylinder is π r2 h. Leave the answer in terms of π.
- Write the formula. Volume = (area of circle) × height = π r2 h.V = π r2 h
- Put in the numbers. The radius is 3 cm and the height is 5 cm.π × 32 × 5
- Work out the powers and multiply. 32 = 9, then multiply by 5.9 × 5 = 45
- Write the answer. 45π cm3
Check. If you use π = 3.14, the volume is about 141 cm3.
Watch out. Square the radius before you multiply by the height.
Method. The curved surface area of a cylinder is 2π r h. Leave the answer in terms of π.
- Write the formula. The curved surface is a rectangle: its length is the circumference, 2π r.CSA = 2π r h
- Put in the numbers. The radius is 3 cm and the height is 5 cm.2 × π × 3 × 5
- Multiply. Multiply the numbers, and keep π.CSA = 30π
- Write the answer. 30π cm2
Check. The curved surface area is the area of the rectangle made by unrolling the curved side.
Method. The volume of a cylinder is π r2 h. Leave the answer in terms of π.
- Write the formula. Volume = (area of circle) × height = π r2 h.V = π r2 h
- Put in the numbers. The radius is 2 cm and the height is 10 cm.π × 22 × 10
- Work out the powers and multiply. 22 = 4, then multiply by 10.4 × 10 = 40
- Write the answer. 40π cm3
Check. If you use π = 3.14, the volume is about 126 cm3.
Watch out. Square the radius before you multiply by the height.
Watch out
Watch out: the curved surface area is 2π r h, and the volume is π r2 h. The volume has the squared radius.
Exam tip
Exam tip. Write the formula you are using before you put in the numbers. It shows the method, even if the arithmetic slips.
Exercise 10D Cylinders
Non-calculatorMethod. The volume of a cylinder is π r2 h. Leave the answer in terms of π.
- Write the formula. Volume = (area of circle) × height = π r2 h.V = π r2 h
- Put in the numbers. The radius is 3 cm and the height is 5 cm.π × 32 × 5
- Work out the powers and multiply. 32 = 9, then multiply by 5.9 × 5 = 45
- Write the answer. 45π cm3
Check. If you use π = 3.14, the volume is about 141 cm3.
Watch out. Square the radius before you multiply by the height.
Method. The volume of a cylinder is π r2 h. Leave the answer in terms of π.
- Write the formula. Volume = (area of circle) × height = π r2 h.V = π r2 h
- Put in the numbers. The radius is 2 cm and the height is 10 cm.π × 22 × 10
- Work out the powers and multiply. 22 = 4, then multiply by 10.4 × 10 = 40
- Write the answer. 40π cm3
Check. If you use π = 3.14, the volume is about 126 cm3.
Watch out. Square the radius before you multiply by the height.
Method. The volume of a cylinder is π r2 h. Leave the answer in terms of π.
- Write the formula. Volume = (area of circle) × height = π r2 h.V = π r2 h
- Put in the numbers. The radius is 4 cm and the height is 3 cm.π × 42 × 3
- Work out the powers and multiply. 42 = 16, then multiply by 3.16 × 3 = 48
- Write the answer. 48π cm3
Check. If you use π = 3.14, the volume is about 151 cm3.
Watch out. Square the radius before you multiply by the height.
Method. The volume of a cylinder is π r2 h. Leave the answer in terms of π.
- Write the formula. Volume = (area of circle) × height = π r2 h.V = π r2 h
- Put in the numbers. The radius is 5 cm and the height is 2 cm.π × 52 × 2
- Work out the powers and multiply. 52 = 25, then multiply by 2.25 × 2 = 50
- Write the answer. 50π cm3
Check. If you use π = 3.14, the volume is about 157 cm3.
Watch out. Square the radius before you multiply by the height.
Method. The volume of a cylinder is π r2 h. Leave the answer in terms of π.
- Write the formula. Volume = (area of circle) × height = π r2 h.V = π r2 h
- Put in the numbers. The radius is 1 cm and the height is 9 cm.π × 12 × 9
- Work out the powers and multiply. 12 = 1, then multiply by 9.1 × 9 = 9
- Write the answer. 9π cm3
Check. If you use π = 3.14, the volume is about 28 cm3.
Watch out. Square the radius before you multiply by the height.
Method. The volume of a cylinder is π r2 h. Leave the answer in terms of π.
- Write the formula. Volume = (area of circle) × height = π r2 h.V = π r2 h
- Put in the numbers. The radius is 6 cm and the height is 2 cm.π × 62 × 2
- Work out the powers and multiply. 62 = 36, then multiply by 2.36 × 2 = 72
- Write the answer. 72π cm3
Check. If you use π = 3.14, the volume is about 226 cm3.
Watch out. Square the radius before you multiply by the height.
Method. The curved surface area of a cylinder is 2π r h. Leave the answer in terms of π.
- Write the formula. The curved surface is a rectangle: its length is the circumference, 2π r.CSA = 2π r h
- Put in the numbers. The radius is 3 cm and the height is 5 cm.2 × π × 3 × 5
- Multiply. Multiply the numbers, and keep π.CSA = 30π
- Write the answer. 30π cm2
Check. The curved surface area is the area of the rectangle made by unrolling the curved side.
Method. The curved surface area of a cylinder is 2π r h. Leave the answer in terms of π.
- Write the formula. The curved surface is a rectangle: its length is the circumference, 2π r.CSA = 2π r h
- Put in the numbers. The radius is 5 cm and the height is 4 cm.2 × π × 5 × 4
- Multiply. Multiply the numbers, and keep π.CSA = 40π
- Write the answer. 40π cm2
Check. The curved surface area is the area of the rectangle made by unrolling the curved side.
Method. The curved surface area of a cylinder is 2π r h. Leave the answer in terms of π.
- Write the formula. The curved surface is a rectangle: its length is the circumference, 2π r.CSA = 2π r h
- Put in the numbers. The radius is 2 cm and the height is 6 cm.2 × π × 2 × 6
- Multiply. Multiply the numbers, and keep π.CSA = 24π
- Write the answer. 24π cm2
Check. The curved surface area is the area of the rectangle made by unrolling the curved side.
Method. The curved surface area of a cylinder is 2π r h. Leave the answer in terms of π.
- Write the formula. The curved surface is a rectangle: its length is the circumference, 2π r.CSA = 2π r h
- Put in the numbers. The radius is 4 cm and the height is 4 cm.2 × π × 4 × 4
- Multiply. Multiply the numbers, and keep π.CSA = 32π
- Write the answer. 32π cm2
Check. The curved surface area is the area of the rectangle made by unrolling the curved side.
Method. The volume of a cylinder is π r2 h. Leave the answer in terms of π.
- Write the formula. Volume = (area of circle) × height = π r2 h.V = π r2 h
- Put in the numbers. The radius is 4 cm and the height is 10 cm.π × 42 × 10
- Work out the powers and multiply. 42 = 16, then multiply by 10.16 × 10 = 160
- Write the answer. 160π cm3
Check. If you use π = 3.14, the volume is about 502 cm3.
Watch out. Square the radius before you multiply by the height.
Method. The curved surface area of a cylinder is 2π r h. Leave the answer in terms of π.
- Write the formula. The curved surface is a rectangle: its length is the circumference, 2π r.CSA = 2π r h
- Put in the numbers. The radius is 3 cm and the height is 12 cm.2 × π × 3 × 12
- Multiply. Multiply the numbers, and keep π.CSA = 72π
- Write the answer. 72π cm2
Check. The curved surface area is the area of the rectangle made by unrolling the curved side.
Method. Use V = π r2 h. Divide by π and by the height, then take the square root.
- Write the formula. V = π r2 h, so 45π = π r2 × 5.45π = 5π r2
- Divide both sides by 5π. This leaves r2.r2 = 9
- Take the square root. The radius is the positive square root of 9.r = 3
- Write the answer. 3 cm
Check. π × 32 × 5 = 45π.
Method. Work out the volume and compare.
- Square the radius. 22 = 4.4
- Multiply by the height. 4 × 3 = 12.12
- Write the answer. The volume is 12π cm3, so Ali is right.Yes
Check. 12π is the volume.
Method. Compare the volumes, using the formula.
- Volume of the first. π × 22 × 5 = 20π.20π
- Volume of the second. π × 42 × 5 = 80π.80π
- Compare. 80 ÷ 20 = 4.4 times
- Write the answer. 4 times
Check. Doubling the radius makes the volume four times bigger, because r is squared.
Surface area of a cuboid and converting units
The surface area of a cuboid is the total area of all six faces. Opposite faces are equal, so find three pairs and add them.
To change units: 1 m = 100 cm. Because area has two dimensions, 1 m2 = 100 × 100 = 10 000 cm2. Because volume has three dimensions, 1 m3 = 100 × 100 × 100 = 1 000 000 cm3.
Key ideas
- Surface area of a cuboid: add the areas of the six faces, in three pairs.
- 1 m2 = 10 000 cm2.
- 1 m3 = 1 000 000 cm3.
- Multiply by 100 for each dimension when changing units of volume.
Method. Find the area of each of the six faces, then add them. Opposite faces are the same.
- Find the area of the top and bottom. Two faces, each 2 × 3.2 × 6 = 12
- Find the area of the front and back. Two faces, each 3 × 4.2 × 12 = 24
- Find the area of the two ends. Two faces, each 2 × 4.2 × 8 = 16
- Add the areas. Add the areas of all six faces.12 + 24 + 16 = 52
- Write the answer. 52 cm2
Check. There are six faces in pairs. Each pair has the same area.
Method. 1 m is 100 cm, so 1 m2 is 100 × 100 = 10 000 cm2. Multiply by 10 000 to change m2 to cm2.
- Write the conversion. 1 m2 = 100 cm × 100 cm.1 m2 = 10 000 cm2
- Multiply. Multiply 2 by 10 000.2 × 10 000 = 20000
- Write the answer. 20,000 cm2
Check. A square metre is a square of side 1 m, which is 100 cm by 100 cm.
Watch out. Do not multiply by 100 for an area. It is 10 000, because there are two dimensions.
Method. Each dimension is 100 times bigger in cm, so multiply by 100 three times.
- Write the conversion. 1 m3 = 100 × 100 × 100 cm3.1 m3 = 1 000 000 cm3
- Multiply. 3 × 1 000 000 = 3 000 000.3 000 000
- Write the answer. 3 000 000 cm3
Check. 1 000 000 ÷ 1 000 000 = 1 m3.
Watch out
Watch out: 1 m2 is 10 000 cm2, not 100 cm2. Multiplying by 100 only works for length, not for area.
Exam tip
Exam tip. When you convert units, write the conversion first, for example 1 m2 = 10 000 cm2. Then multiply.
Exercise 10E Surface area and units
Non-calculatorMethod. Find the area of each of the six faces, then add them. Opposite faces are the same.
- Find the area of the top and bottom. Two faces, each 2 × 3.2 × 6 = 12
- Find the area of the front and back. Two faces, each 3 × 4.2 × 12 = 24
- Find the area of the two ends. Two faces, each 2 × 4.2 × 8 = 16
- Add the areas. Add the areas of all six faces.12 + 24 + 16 = 52
- Write the answer. 52 cm2
Check. There are six faces in pairs. Each pair has the same area.
Method. Find the area of each of the six faces, then add them. Opposite faces are the same.
- Find the area of the top and bottom. Two faces, each 5 × 5.2 × 25 = 50
- Find the area of the front and back. Two faces, each 5 × 2.2 × 10 = 20
- Find the area of the two ends. Two faces, each 5 × 2.2 × 10 = 20
- Add the areas. Add the areas of all six faces.50 + 20 + 20 = 90
- Write the answer. 90 cm2
Check. There are six faces in pairs. Each pair has the same area.
Method. Find the area of each of the six faces, then add them. Opposite faces are the same.
- Find the area of the top and bottom. Two faces, each 10 × 4.2 × 40 = 80
- Find the area of the front and back. Two faces, each 4 × 3.2 × 12 = 24
- Find the area of the two ends. Two faces, each 10 × 3.2 × 30 = 60
- Add the areas. Add the areas of all six faces.80 + 24 + 60 = 164
- Write the answer. 164 cm2
Check. There are six faces in pairs. Each pair has the same area.
Method. Find the area of each of the six faces, then add them. Opposite faces are the same.
- Find the area of the top and bottom. Two faces, each 6 × 2.2 × 12 = 24
- Find the area of the front and back. Two faces, each 2 × 1.2 × 2 = 4
- Find the area of the two ends. Two faces, each 6 × 1.2 × 6 = 12
- Add the areas. Add the areas of all six faces.24 + 4 + 12 = 40
- Write the answer. 40 cm2
Check. There are six faces in pairs. Each pair has the same area.
Method. Find the area of each of the six faces, then add them. Opposite faces are the same.
- Find the area of the top and bottom. Two faces, each 3 × 3.2 × 9 = 18
- Find the area of the front and back. Two faces, each 3 × 3.2 × 9 = 18
- Find the area of the two ends. Two faces, each 3 × 3.2 × 9 = 18
- Add the areas. Add the areas of all six faces.18 + 18 + 18 = 54
- Write the answer. 54 cm2
Check. There are six faces in pairs. Each pair has the same area.
Method. Find the area of each of the six faces, then add them. Opposite faces are the same.
- Find the area of the top and bottom. Two faces, each 4 × 2.2 × 8 = 16
- Find the area of the front and back. Two faces, each 2 × 7.2 × 14 = 28
- Find the area of the two ends. Two faces, each 4 × 7.2 × 28 = 56
- Add the areas. Add the areas of all six faces.16 + 28 + 56 = 100
- Write the answer. 100 cm2
Check. There are six faces in pairs. Each pair has the same area.
Method. 1 m is 100 cm, so 1 m2 is 100 × 100 = 10 000 cm2. Multiply by 10 000 to change m2 to cm2.
- Write the conversion. 1 m2 = 100 cm × 100 cm.1 m2 = 10 000 cm2
- Multiply. Multiply 2 by 10 000.2 × 10 000 = 20000
- Write the answer. 20,000 cm2
Check. A square metre is a square of side 1 m, which is 100 cm by 100 cm.
Watch out. Do not multiply by 100 for an area. It is 10 000, because there are two dimensions.
Method. 1 m is 100 cm, so 1 m2 is 100 × 100 = 10 000 cm2. Multiply by 10 000 to change m2 to cm2.
- Write the conversion. 1 m2 = 100 cm × 100 cm.1 m2 = 10 000 cm2
- Multiply. Multiply 0.5 by 10 000.0.5 × 10 000 = 5000
- Write the answer. 5,000 cm2
Check. A square metre is a square of side 1 m, which is 100 cm by 100 cm.
Watch out. Do not multiply by 100 for an area. It is 10 000, because there are two dimensions.
Method. 1 m is 100 cm, so 1 m2 is 100 × 100 = 10 000 cm2. Multiply by 10 000 to change m2 to cm2.
- Write the conversion. 1 m2 = 100 cm × 100 cm.1 m2 = 10 000 cm2
- Multiply. Multiply 3 by 10 000.3 × 10 000 = 30000
- Write the answer. 30,000 cm2
Check. A square metre is a square of side 1 m, which is 100 cm by 100 cm.
Watch out. Do not multiply by 100 for an area. It is 10 000, because there are two dimensions.
Method. Each dimension is 100 times bigger in cm, so the volume is 100 × 100 × 100 times bigger.
- Write the conversion. 1 m3 = 1 000 000 cm3.1 000 000
- Multiply. 3 × 1 000 000 = 3 000 000.3 000 000
- Write the answer. 3 000 000 cm3
Check. Divide back: 3 000 000 ÷ 1 000 000 = 3.
Method. Change every length to cm, then multiply.
- Change the length. 1 m = 100 cm.100 cm
- Multiply. 100 × 50 × 20 = 100 000.100 000
- Write the answer. 100 000 cm3
Check. 100 000 cm3 is 0.1 m3.
Method. Find the area of each of the six faces, then add them. Opposite faces are the same.
- Find the area of the top and bottom. Two faces, each 4 × 3.2 × 12 = 24
- Find the area of the front and back. Two faces, each 3 × 2.2 × 6 = 12
- Find the area of the two ends. Two faces, each 4 × 2.2 × 8 = 16
- Add the areas. Add the areas of all six faces.24 + 12 + 16 = 52
- Write the answer. 52 cm2
Check. There are six faces in pairs. Each pair has the same area.
Method. 1 m is 100 cm, so 1 m2 is 100 × 100 = 10 000 cm2. Multiply by 10 000 to change m2 to cm2.
- Write the conversion. 1 m2 = 100 cm × 100 cm.1 m2 = 10 000 cm2
- Multiply. Multiply 5 by 10 000.5 × 10 000 = 50000
- Write the answer. 50,000 cm2
Check. A square metre is a square of side 1 m, which is 100 cm by 100 cm.
Watch out. Do not multiply by 100 for an area. It is 10 000, because there are two dimensions.
Method. Check the conversion for area.
- Write the conversion for length. 1 m = 100 cm.100 cm
- Square it for area. 1 m2 = 100 × 100 = 10 000 cm2.10 000
- Write the answer. Ali is not right. 1 m2 = 10 000 cm2.No
Check. 100 cm2 is only a square 10 cm by 10 cm.
Method. Find the surface area of each cuboid.
- Work out the first. The cuboid 2 by 3 by 4 has surface area 2(6 + 12 + 8) = 52 cm2.52
- Find another cuboid with volume 24. A 1 by 4 by 6 cuboid also has volume 24, with surface area 2(4 + 24 + 6) = 68 cm2.68
- Compare. The surface areas are different.52 ≠ 68
- Write the answer. Ali is not right: the same volume can have different surface areas.No
Check. Same volume, different shape, different surface area.
Chapter summary
| Skill | Method |
|---|---|
| Trapezium | (a + b) ÷ 2 × h |
| Circle | Area π r2; circumference 2π r |
| Prism | Volume = area of cross-section × length |
| Cylinder | Volume π r2 h; curved surface area 2π r h |
| Cuboid | Volume = l × w × h; surface area = 2(lw + wh + lh) |
| Units | 1 m2 = 10 000 cm2; 1 m3 = 1 000 000 cm3 |
Can you…?
- I can find the area of a trapezium and of a compound shape
- I can find the area and circumference of a circle, in terms of π
- I can find the volume of a cuboid and of a prism
- I can find the volume and curved surface area of a cylinder, in terms of π
- I can find the surface area of a cuboid
- I can convert units of area and volume
Key terms
- trapezium
- a quadrilateral with one pair of parallel sides
- prism
- a solid with the same cross-section all the way along its length
- cross-section
- the shape you see when a solid is cut straight across
- cylinder
- a prism with a circular cross-section
- surface area
- the total area of all the faces of a solid
- volume
- the amount of space a solid takes up
- pi
- the ratio of the circumference of a circle to its diameter, written as π
Exercise 10Z Check-up
Non-calculatorMethod. The area of a trapezium is the average of the two parallel sides, times the height.
- Write the formula. Area = (a + b) ÷ 2 × h.A = (a + b) ÷ 2 × h
- Add the parallel sides. The parallel sides are 6 cm and 10 cm.6 + 10 = 16
- Halve and multiply by the height. Half of 16 is 8, then multiply by 5 cm.8 × 5 = 40
- Write the answer. 40 cm2
Check. The average of 6 and 10 is 8.0, and 8.0 × 5 = 40.0.
Watch out. Use the perpendicular height, not the slanted side.
Method. The area of a circle is π r2. Leave the answer in terms of π.
- Write the formula. Area = π r2.A = π r2
- Square the radius. The radius is 9 cm, so r2 = 9 × 9.92 = 81
- Multiply by π. Leave π in the answer.A = 81π
- Write the answer. 81π cm2
Check. If you use π = 3.14, the area is about 254 cm2.
Watch out. Square the radius first, not the diameter. The diameter is twice the radius.
Method. The circumference of a circle is 2π r. Leave the answer in terms of π.
- Write the formula. Circumference = 2π r.C = 2π r
- Put in r. The radius is 6 cm.2 × π × 6
- Multiply. Multiply the numbers, and keep π.C = 12π
- Write the answer. 12π cm
Check. The circumference is about 3 times the diameter (12 cm), so about 36 cm.
Watch out. Use the radius with the formula 2πr, or the diameter with πd.
Method. The volume of a cuboid is length × width × height.
- Write the formula. Volume = length × width × height.V = l × w × h
- Put in the numbers. The sides are 6 cm, 5 cm and 2 cm.6 × 5 × 2
- Multiply. Multiply the three numbers.30 × 2 = 60
- Write the answer. 60 cm3
Check. Check: 60 ÷ 2 = 30, which is the area of the base.
Watch out. Write the answer in cubic units, cm3.
Method. For a prism, the volume is the area of the cross-section × the length.
- Find the area of the cross-section. This is the area of the end face of the prism.area = 15 cm2
- Multiply by the length. The prism is 8 cm long.15 × 8 = 120
- Write the answer. 120 cm3
Check. Check: 120 ÷ 8 = 15, the cross-section area.
Watch out. Use the area of the cross-section (the end), not the area of the long face.
Method. The volume of a cylinder is π r2 h. Leave the answer in terms of π.
- Write the formula. Volume = (area of circle) × height = π r2 h.V = π r2 h
- Put in the numbers. The radius is 2 cm and the height is 7 cm.π × 22 × 7
- Work out the powers and multiply. 22 = 4, then multiply by 7.4 × 7 = 28
- Write the answer. 28π cm3
Check. If you use π = 3.14, the volume is about 88 cm3.
Watch out. Square the radius before you multiply by the height.
Method. The curved surface area of a cylinder is 2π r h. Leave the answer in terms of π.
- Write the formula. The curved surface is a rectangle: its length is the circumference, 2π r.CSA = 2π r h
- Put in the numbers. The radius is 4 cm and the height is 3 cm.2 × π × 4 × 3
- Multiply. Multiply the numbers, and keep π.CSA = 24π
- Write the answer. 24π cm2
Check. The curved surface area is the area of the rectangle made by unrolling the curved side.
Method. Find the area of each of the six faces, then add them. Opposite faces are the same.
- Find the area of the top and bottom. Two faces, each 5 × 2.2 × 10 = 20
- Find the area of the front and back. Two faces, each 2 × 3.2 × 6 = 12
- Find the area of the two ends. Two faces, each 5 × 3.2 × 15 = 30
- Add the areas. Add the areas of all six faces.20 + 12 + 30 = 62
- Write the answer. 62 cm2
Check. There are six faces in pairs. Each pair has the same area.
Method. 1 m is 100 cm, so 1 m2 is 100 × 100 = 10 000 cm2. Multiply by 10 000 to change m2 to cm2.
- Write the conversion. 1 m2 = 100 cm × 100 cm.1 m2 = 10 000 cm2
- Multiply. Multiply 4 by 10 000.4 × 10 000 = 40000
- Write the answer. 40,000 cm2
Check. A square metre is a square of side 1 m, which is 100 cm by 100 cm.
Watch out. Do not multiply by 100 for an area. It is 10 000, because there are two dimensions.
Method. Use the formula backwards: A = (a + b) ÷ 2 × h.
- Write the formula. 30 = (5 + 7) ÷ 2 × h.30 = 6h
- Solve. Divide both sides by 6.h = 5
- Write the answer. 5 cm
Check. (5 + 7) ÷ 2 × 5 = 30.
Total 31 marks. Allow about 31 minutes.
Exercise 10X Exam-style questions
Non-calculatorMethod. The area of a trapezium is the average of the two parallel sides, times the height.
- Write the formula. Area = (a + b) ÷ 2 × h.A = (a + b) ÷ 2 × h
- Add the parallel sides. The parallel sides are 7 cm and 13 cm.7 + 13 = 20
- Halve and multiply by the height. Half of 20 is 10, then multiply by 6 cm.10 × 6 = 60
- Write the answer. 60 cm2
Check. The average of 7 and 13 is 10.0, and 10.0 × 6 = 60.0.
Watch out. Use the perpendicular height, not the slanted side.
Method. The circumference of a circle is 2π r. Leave the answer in terms of π.
- Write the formula. Circumference = 2π r.C = 2π r
- Put in r. The radius is 5 cm.2 × π × 5
- Multiply. Multiply the numbers, and keep π.C = 10π
- Write the answer. 10π cm
Check. The circumference is about 3 times the diameter (10 cm), so about 30 cm.
Watch out. Use the radius with the formula 2πr, or the diameter with πd.
Method. For a prism, the volume is the area of the cross-section × the length.
- Find the area of the cross-section. This is the area of the end face of the prism.area = 9 cm2
- Multiply by the length. The prism is 12 cm long.9 × 12 = 108
- Write the answer. 108 cm3
Check. Check: 108 ÷ 12 = 9, the cross-section area.
Watch out. Use the area of the cross-section (the end), not the area of the long face.
Method. The volume of a cylinder is π r2 h. Leave the answer in terms of π.
- Write the formula. Volume = (area of circle) × height = π r2 h.V = π r2 h
- Put in the numbers. The radius is 3 cm and the height is 8 cm.π × 32 × 8
- Work out the powers and multiply. 32 = 9, then multiply by 8.9 × 8 = 72
- Write the answer. 72π cm3
Check. If you use π = 3.14, the volume is about 226 cm3.
Watch out. Square the radius before you multiply by the height.
Method. The curved surface area of a cylinder is 2π r h. Leave the answer in terms of π.
- Write the formula. The curved surface is a rectangle: its length is the circumference, 2π r.CSA = 2π r h
- Put in the numbers. The radius is 5 cm and the height is 4 cm.2 × π × 5 × 4
- Multiply. Multiply the numbers, and keep π.CSA = 40π
- Write the answer. 40π cm2
Check. The curved surface area is the area of the rectangle made by unrolling the curved side.
Method. Find the area of each of the six faces, then add them. Opposite faces are the same.
- Find the area of the top and bottom. Two faces, each 4 × 3.2 × 12 = 24
- Find the area of the front and back. Two faces, each 3 × 10.2 × 30 = 60
- Find the area of the two ends. Two faces, each 4 × 10.2 × 40 = 80
- Add the areas. Add the areas of all six faces.24 + 60 + 80 = 164
- Write the answer. 164 cm2
Check. There are six faces in pairs. Each pair has the same area.
Method. Multiply length, width and depth.
- Multiply. 10 × 5 × 2 = 100.100
- Write the answer. 100 m3
Check. 100 ÷ 2 = 50, which is the area of the surface.
Method. Multiply by 1 000 000, because 1 m3 = 1 000 000 cm3.
- Write the conversion. 1 m3 = 1 000 000 cm3.1 000 000
- Multiply. 2.5 × 1 000 000 = 2 500 000.2 500 000
- Write the answer. 2 500 000 cm3
Check. 2 500 000 ÷ 1 000 000 = 2.5.
Method. The volume of a cylinder is π r2 h. Leave the answer in terms of π.
- Write the formula. Volume = (area of circle) × height = π r2 h.V = π r2 h
- Put in the numbers. The radius is 4 cm and the height is 10 cm.π × 42 × 10
- Work out the powers and multiply. 42 = 16, then multiply by 10.16 × 10 = 160
- Write the answer. 160π cm3
Check. If you use π = 3.14, the volume is about 502 cm3.
Watch out. Square the radius before you multiply by the height.
Method. The cross-section is a triangle. Find its area, then multiply by the length.
- Find the area of the cross-section. ½ × 8 × 5 = 20 cm2.20
- Multiply by the length. 20 × 6 = 120.120
- Write the answer. 120 cm3
Check. 120 ÷ 6 = 20.
Method. Write the surface area as an equation in h, then solve it.
- Write the surface area. 2(2 × 3 + 3h + 2h) = 52.2(6 + 5h) = 52
- Divide both sides by 2. 6 + 5h = 26.6 + 5h = 26
- Solve. 5h = 20, so h = 4.h = 4
- Write the answer. 4 cm
Check. 2(6 + 20) = 52.