In an equilateral triangle ABC ,D is a point on side BC such that BD=1/3BC. Prove that. 9AD^2=7AB^2

In an equilateral triangle ABC ,D is a point on side BC such that BD=1

1.	In an equilateral triangle ABC ,D is a point on side BC such that BD=1/3BC. Prove that. 9AD^2=7AB^2
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