http://i.imgur.com/dCp7iuh.png can anyone help me solving this please?

Question

mertsj · Answer

I can tell what the problem means.  It is partly in  a foreign language.

mertsj · Answer

*can't

anonymous · Answer

The question is "Prove the identity", we need to prove that cos2a + cos2b + cos2g = -4sinAsinBSinG - 1

mertsj · Answer

So this is the problem?
$$\cos 2A+\cos 2B+\cos 2C=-4\sin A \sin B \sin C - 1$$

anonymous · Answer

Yes, we need to prove that both sides mean the same thing.

mertsj · Answer

What have you tried so far?

anonymous · Answer

To be honest, I almost don't have idea how to solve it, the best I have tried is http://i.imgur.com/2yk3w1g.png, Am not even sure if its right.

mertsj · Answer

There must be more to the problem.  Let A=30, B=35 and C = 40 and you will see the "identity" is NOT true.

anonymous · Answer

well,: A + B + C = 180

mertsj · Answer

Did you try it?  It is not true.  Try A=80, B= 60, C=40

anonymous · Answer

well, our teacher proved it somehow before few months, but the problem is that none remembers and we got exam tomorow. The question is to prove that cos2A+cos2B+cos2C=−4sinAsinBsinC−1, IF A + B + C = 180