18. A circle with center $C$ and radius $r$ intersects the square $E F G H$ at $H$ and at $M$, the midpoint of $E F$. If $C, E$ and $F$ are collinear and $E$ lies between $C$ and $F$, what is the area of the region outside the circle and inside the square in terms of $r$ ?
18. $r^{2}\left(\frac{22}{25}-\frac{\tan ^{-1}(4 / 3)}{2}\right)$