How to find the area of a square - GRE Quantitative Reasoning

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Question

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In the figure above, the circle is inscribed within the bounding square. If r = 5, what is the area of the shaded region?

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Answer

In order to solve this, we must first find the area of the containing square and then remove the inscribed circle. Once this is done, we need to divide our result by 4 in order to get the one-forth that is the one shaded region.

One side of the square will be equal to the circle's diameter (2r). Since r = 5, d = 10. Therefore, the area of the square is d2 = 102 = 100. The area of the circle is πr2 = 52π = 25π.

Therefore, the area of the four "corner regions" is equal to 100 - 25π. One of these is equal to (100 - 25π) / 4. Simplified, this is 25 - (25/4)π.

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