hi mukesh,

Here's an outline of a way to prove this. see diagram below.

There's no right angle to get tanA easily so I used the sine and cosine rules:

and

Put these together to get tanA and simplify.

work on this expression for tanA, making use of the following:

After much simplification you can get this equal to -2tanB, from which the required result follows.

It's a tough one so expect it to take 2 or 3 pages. If you get stuck post back where you've got to, and I'll compare your answer with mine.

Bob