GRE Math : How to find the area of a circle

Study concepts, example questions & explanations for GRE Math

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Example Questions

Example Question #13 : Geometry

Circle B has a circumference of 36π. What is the area of circle A, which has a radius half the length of the radius of circle B?

Possible Answers:

81π

18π

324π

18

Correct answer:

81π

Explanation:

To find the radius of circle B, use the circumference formula (c = πd = 2πr):

2πr = 36π

Divide each side by 2π: r = 18

Now, if circle A has a radius half the length of that of B, A's radius is 18 / 2 = 9.

The area of a circle is πr2.  Therefore, for A, it is π*92 = 81π.

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