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Math Help - Adding trig functions

  1. #1
    Member
    Joined
    Sep 2009
    Posts
    181

    Adding trig functions

    Hello,

    Can someone please tell me how to add trig functions?

    for example, what do you add cos2x? is it cosx+cosx = cos2x?
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  2. #2
    MHF Contributor
    Joined
    Apr 2008
    Posts
    1,092
    Using the identity

    \cos(a + b) = \cos a \cos b - \sin a \sin b,

    we have

    \cos(2x) = \cos(x + x) = \cos^2x - \sin^2x.

    Then, using the identity

    \sin^2x + \cos^2x = 1

    we have

    \cos(2x) = 2 \cos^2x - 1 = 1 - 2\sin^2x
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