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  • I have a 10cm radius circle. I want to have another one inside it such that the area of the inner circle is equal to the area of the outer 'donut' section. Presumably it's not 5cm, but what is it?

  • If the small circle is r and the large one R, then the area of the small circle is half the area of the large circle when r=(R2/2)0.5

    In this case R=10 so r=7.071

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