In ABC areas of CGB, ar(CGA) and ar(AGB) are equal., median of a triangle divides it into triangles of equal areas.
=CGA=CGB=AGB .......(i)
=ar(CGA+CGB+AGB) = area of tr ABC.
From (i)3(area of AGB) = area of ABC.
=ar(triangle GAB) = 1/3 * area of triangle ABC