(sorry,can't post pics,somethings wrong with my browser and my english is also shaky)

if n diameters are drawn in a circle than 2n triangles are drawn,n on each side of a specific diameter.for every triangle there is a congruent triangle on other side(2 sides are radii,interior angles are vertical) thus n triangles on one side are congruent to n on the other side.as n tends to infinity the area becomes equal to other sides area.so, both sides have equal area.