Calculate, correct to 3 significant figures, the circumference of a circle with:

1. (i) radius 7 cm
2. (ii) radius 10 cm
3. (iii) radius 12 cm

Step-by-step solution

Idea: Use C = 2πr with π = 227, then round to 3 significant figures (count from the first non-zero digit).

Check: With π = 3.1416 the values are 43.98, 62.83 and 75.40 cm. The 227 answers are very slightly larger, because 227 is slightly more than π.

Answer to write in the exam

Common mistakes that cost marks

• Writing 44 cm in (i). To 3 significant figures it is 44.0 cm.
• Rounding 62.857 to 62.8 (cutting off) instead of 62.9.
• Using πr instead of 2πr.

How this can come in the exam

Try one yourself

Find, correct to 3 significant figures, the circumference of a circle of radius 9 cm. (Use π = 227.)

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