year 8 maths

126 Chapter 3 Measurement and Pythagoras’ theorem
Example 4
Finding the circumference with a calculator
Find the circumference of these circles, correct to two decimal places. Use a calculator for the value of π .
a
b
3.5 m
4 cm
SOLUTION
a
b
C = 2πr
= 2 × π × 3.5
= 7π
= 21.99 m (to 2 d.p.)
C = πd
= π × 4
= 4π
= 12.57 cm (to 2 d.p.)
EXPLANATION
Since r is given, you can use C = 2πr .
Alternatively, use C = πd with d = 7 .
Round off, as instructed.
Substitute d = 4 into the rule C = πd or use
C = 2πr with r = 2 .
Round off, as instructed.
Example 5
Finding the circumference without a calculator
Calculate the circumference of these circles, using the given approximation of π .
a
b
10 m
14 cm
π = 3.14
π = 22 7
SOLUTION
a
b
C = πd
= 3.14 × 10
= 31.4 m
C = 2πr
= 2 × 22 7 × 14
EXPLANATION
Use π = 3.14 and multiply mentally. Move the
decimal point one place to the right.
Alternatively, use C = 2πr with r = 5 .
Use π = 22 and cancel the 14 with the 7 before
7
calculating the final answer.
= 88 cm
2 × 22 7
× 14 = 2 × 22 × 2
Cambridge Maths NSW
Stage 4 Year 8 Second edition
ISBN 978-1-108-46627-1 © Palmer et al. 2018
Cambridge University Press
Photocopying is restricted under law and this material must not be transferred to another party.