# Math Help - Help in trignometry question

1. ## Help in trignometry question

Could anyone please tell me how can I prove the following two trignometry question?

1) cos^2 A + cos^2 B - 2 cos A cos B cos(A + B) = sin^2 (A + B)

2) tan 8A - tan 6A - tan 2B = tan 8A tan 6A tan 2A

2. Originally Posted by pawan322
Could anyone please tell me how can I prove the following two trignometry question?

1) cos^2 A + cos^2 B - 2 cos A cos B cos(A + B) = sin^2 (A + B)

2) tan 8A - tan 6A - tan 2B = tan 8A tan 6A tan 2A
For Q1, I'd start with the RHS using the expansion for sin(A+B) which you should know, and then square it.
Then attack the LHS by using the expansion for cos(A+B), which you should also know.
Hopefully after you've done that, both will be the same. LHS=RHS QED

Give it a go!

3. Thanks for your reply. Could you please tell me how can I solve the second question?

4. Originally Posted by pawan322