$\displaystyle \frac{1}{2}r^2$\displaystyle \frac{1}{2}r^2

$\displaystyle \frac{1}{4}r^2$\displaystyle \frac{1}{4}r^2

$\displaystyle \frac{\pi}{4}r^2$\displaystyle \frac{\pi}{4}r^2

$\displaystyle \frac{\sqrt{3}}{4}r^2$\displaystyle \frac{\sqrt{3}}{4}r^2