Worked Example 1
Find the area of a circle of radius 21 cm. Take pi as 22/7.
Step 1: square the radius firstr² = 21 × 21 = 441
Step 2: multiply by piA = 22/7 × 441 = 22 × 63 = 1386 cm²
Why the question chose 21Radii of 7, 14, 21 and 28 are chosen so that the 7 in 22/7 cancels neatly.
Worked Example 2
A circular table top is 1.4 m across. Find the cost of polishing it at Rs 450 per square metre.
Across means diameterd = 1.4 m, so r = 0.7 m.
AreaA = 22/7 × 0.49 = 1.54 m²
Cost1.54 × 450 = Rs 693
Sense checkThe table would just fit inside a 1.4 m square of area 1.96 m². Our 1.54 is a bit less, which is right: a circle fills a little over three quarters of the square around it.
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