If bisector of angle of a triangle bisects the opposite side, prove that the triangle is isosceles
O is a point in the interior of a triangle abc,od perpendicular bc,oe perpendicular ac and of perpendicular ab. Show that 1.oa^2+ob^2+oc^2-od^2-oe^2-of^2 =af^2 +bd^2+ce^2