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: The circle has radius $1$. By symmetry (join the centre of the circle to the other corners of the square) the shaded part of the circle is one-quarter of the whole circle. 
