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Math Help - Small trigonometry question

  1. #1
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    Small trigonometry question

    How would I go about solving the equation

    cos x = cos 3x ?

    I know the equation for cos 2x but this doesn't help - I get 2cos^2(3x/2) - 1 which doesn't really help. Is there any other way which it can be done?
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  2. #2
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    Quote Originally Posted by db5vry View Post
    How would I go about solving the equation

    cos x = cos 3x ?

    I know the equation for cos 2x but this doesn't help - I get 2cos^2(3x/2) - 1 which doesn't really help. Is there any other way which it can be done?
    Sure db5vry,

    express Cos(3x) as Cos(2x+x)

    expand this using Cos(A+B) and expand Cos(2x) later using Cos(x+x).
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  3. #3
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    or turn \cos x-\cos3x by using sum - product formulae.
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