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

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- May 1st 2010, 01:43 AMpawan322Help 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 - May 1st 2010, 02:11 AMDebsta
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! - May 1st 2010, 03:07 AMpawan322
Thanks for your reply. Could you please tell me how can I solve the second question?

- May 1st 2010, 03:20 AMDebsta