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Math Help - [SOLVED] Use these equalities to derive the following trig identities.

  1. #1
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    [SOLVED] Use these equalities to derive the following trig identities.

    (A) sin(-x) = -sinx
    (B) cos(-x) = cosx
    (C) cos(x+y) = cosx cosy + sinx siny
    (D) sin(x+y) = sinx cosy + cosx siny

    I'm stuck on the very first question:

    (a) sin²x + cos²x = 1 (use (C) and cos0 = 1)

    the answers also say to use (A) and (B).. but I dunno how that would apply. It also said to use x = y. How am I supposed to use cos0 though?

    thanks in advance for the help!
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  2. #2
    Super Member Random Variable's Avatar
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     \cos (x-y) = \sin x \sin y + \cos x \cos y

     \sin^{2} x + \cos^{2} x = \sin x \sin x + \cos x \cos x = \cos (x-x)= \cos 0 =1
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