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## Question

primary 5 | Maths
| Rate & Ratio

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how to do, tks

Combined area of the two smaller semicircles

= 3.14 x 4.5cm x 4.5cm

= 63.585cm²

Combined area of the two larger semicircles

= 3.14 x 6cm x 6cm

= 113.04cm²

Area of rectangle

= 9cm x 12cm

= 108cm²

Notice the large unshaded circle in the middle. See the 15cm line? That's its diameter. So its radius is 7.5cm.

Area of large unshaded circle

= 3.14 x 7.5cm x 7.5cm

= 176.625cm²

Shaded area

= Area of all 4 semicircles + area of rectangle - area of large unshaded circle

= 63.585cm² + 113.04cm² + 108cm² - 176.625cm²

= 108cm²

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Also, there is a calculation error in your second last step. It should be 176.625cm² instead of 149.625cm².

Notice that the 15cm line cuts across the centre of the large unshaded circle. So the diameter is 15cm and therefore radius is 7.5cm

Tip :

Two semicircles of the same size can be combined into a single circle. So in your working, you can just write

½ x 3.14 x 6 x 6 x 2

Or

3.14 x 6 x 6

in a single step instead of dividing by ½ to find one semicircle and then multiplying by 2 again