Hi Maithreyi.
A full circle is 360 degrees and its area is π r^{2} (where r is the radius). So a 180 degree portion of the circle is 180/360 = 1/2 of the area of the full circle. So all you need to do is find the area of the full circle first, then determine what portion of the full circle each sector represents by using a fraction.
Hope this helps,
Stephen La Rocque.
