In an equilateral triangle ABC ,D is a point on side BC such that BD= 1\\3BC prove that 9AD2 = 7AB2

In an equilateral triangle ABC ,D is a point on side BC such that BD=

1.	In an equilateral triangle ABC ,D is a point on side BC such that BD= 1\\3BC prove that 9AD2 = 7AB2
Answer» RD Sharma pg no..4.112 ex.25