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Math Help - Integration/trig identities

  1. #1
    haydenr
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    Integration/trig identities

    How can i solve the indef. Integral 4Cos[4x]Cos[x]dx

    Or, maybe which identities does this involve? Thanks!
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  2. #2
    Grand Panjandrum
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    Quote Originally Posted by haydenr View Post
    How can i solve the indef. Integral 4Cos[4x]Cos[x]dx

    Or, maybe which identities does this involve? Thanks!
    Use the trig identity:

    2cos(A)cos(B) = cos(A+B) + cos(A-B)

    on the integrand to get:

    Int 4 cos(4x) cos(x) dx = Int 2 cos(5x) dx + Int 2 cos(3x) dx

    RonL
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