A ball bearing has a radius of 0.7 cm. Find its surface area.

Step-by-step solution

Idea: A ball bearing is a sphere: surface area = 4πr². With 0.7 = 7 ÷ 10, π = 227 cancels neatly.

1. Surface area = 4πr² = 4 × 227 × 0.7 × 0.7.1 mark
2. 227 × 0.7 = 2.2, so 4 × 2.2 × 0.7 = 6.16 cm².1 mark

Check: With π = 3.14: 4 × 3.14 × 0.49 = 6.15 cm² ≈ 6.16 ✓.

Answer to write in the exam

Common mistakes that cost marks

• Using 2πr² (the hemisphere’s curved surface).
• Writing 0.7 × 0.7 = 4.9 instead of 0.49.

How this can come in the exam

Try one yourself

Find the surface area of a sphere of radius 10.5 cm. (π = 227)

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