The shaded region is actually a sector and the formula for the area of a sector is:
central angle/360 x pi x radius squared, which is basically a fraction of the area of the circle. With this basic background knowledge, all we have to do is substitute the values into the formula.
(80/36) x pi x 81
= 80/4 x pi x 9
= 720/4 x pi
= 180pi cm squared.
