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Math Help - Help in trignometry question

  1. #1
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    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
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  2. #2
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    Quote Originally Posted by pawan322 View Post
    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!
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  3. #3
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    Thanks for your reply. Could you please tell me how can I solve the second question?
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  4. #4
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    Quote Originally Posted by pawan322 View Post
    Thanks for your reply. Could you please tell me how can I solve the second question?
    I'd probably use the tan (double angle) rule to express tan 8x in terms of tan 4x.
    Then try using tan(A+B) rule to express tan 6x in terms of tan 4x and tan 2x.
    Then hopefully it will all come together.

    (I'm assuming you've got a B where you mean an A??)
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