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  1. May 13th 2009, 12:54 PM #1
    IronMan21
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    If Cos(x)=3/5, what is Cos(3x) equal to?
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  2. May 13th 2009, 01:01 PM #2
    e^(i*pi)
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    Quote Originally Posted by IronMan21 View Post
    If Cos(x)=3/5, what is Cos(3x) equal to?
    cos(3x) = cos(2x+x) = cos(2x)cos(x) - sin(2x)sin(x)

    = (2cos^2(x)-1)(cos(x)) - 2sin^2(x)cos(x) = 2cos^3(x) - cos(x) - (2cos(x) - 2cos^3(x))

    = 2cos^3(x) - cos(x) - 2cos(x) + 2cos^3(x) = 4cos^3(x) - 3cos(x)

    Now sub in your value for cos(x) and solve
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