Given an isosceles triangle ABC, there is BA = BC = a, AC = b. The angle bisector A intersects BC at M, the angle bisector C intersects BA at N

 Given an isosceles triangle ABC, there is BA = BC = a, AC = b. The angle bisector A intersects BC at M, the angle bisector C intersects BA at N

a) Cm: MN // AC
B)mn=???