Let 'r' represent the radius of the semicircle.

The perimeter of a circle is: P = 2*r*pi

The perimeter of a semi-circle is: (1/2)*P = (1/2)*2*r*pi = r*pi

The increased radius by 9 is: r + 9

The perimeter of a semicircle is doubled when the radius is increased by 9:

(r + 9)*pi = 2*r*pi divide both sides by pi

r + 9 = 2r

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click here to see the radius calculation

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r = 9

The radius of the semicircle is 9.