The area of circle b is 36 times greater than the area of circle
a. if the radius of circle b is 12 feet, what is the radius of circle a?

The formula for area of a circle is pi r^2.
Circle A= pi a^2
Circle B= pi b^2

Now if the area of circle B is 36 times greater than the area of circle A, we can translate than into:
areaB=36*areaA
pi b^2 = 36 pi a^2
pi b^2 = 36 pi 12^2 (sub in variables)
b^2 = 36*12^2 (divide pi from both sides)
b^2 = 36 * 144 (solve 12^2)
b^2 = 5184 (simplify right hand side)
b = 72 (take square root on both sides)
Your answer is 72. 

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