How do I find the area of a circle inside a square? 
In the figure above, the circle with center $O$ is inscribed in square $ABCD$. What is the area of the shaded portion of the circle?
(A) $\pi/4$
(B) $\pi/2$
(C) $\pi$
(D) $3\pi/2$
(E) $2\pi$

Hint:
Radius of the circle is ...
Area of the whole disc is equal ...
What part of the disc is shaded?
Therefore, area of the shaded part is equal ...
