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Revision as of 21:24, 3 August 2006

Problem

A square of area 40 is inscribed in a semicircle as shown. What is the area of the semicircle?

2006amc10b08.gif

$\mathrm{(A) \ } 20\pi\qquad \mathrm{(B) \ } 25\pi\qquad \mathrm{(C) \ } 30\pi\qquad \mathrm{(D) \ } 40\pi\qquad \mathrm{(E) \ } 50\pi$

Solution

Since the area of the square is $40$, the length of the side is $\sqrt{40}=2\sqrt{10}$. The distance between the center of the semicircle and one of the bottom vertecies of the square is half the length of the side, which is $\sqrt{10}$.

Using the Pythagorean Theorem to find the square of radius:

$(2\sqrt{10})^2 + (\sqrt{10})^2 = r^2$

$50=r^2$

So, the area of the semicircle is $\frac{1}{2}\cdot \pi \cdot 50 = 25\pi \Rightarrow B$

See Also

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