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Question
primary 5 | Maths
| Rate & Ratio
One Answer Below
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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
See 1 Answer
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