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math help

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I really don't understand how to write the expression as one invloving only sinϴ and cosϴ


  • math help -

    first of all you should by now have proven that
    sin 2x = 2sinxcosx

    by writing cos 3x as cos(2x + x), expanding that and then changing the cos 2x and sin 2x factors you can show that
    cos 3x = cos^ 3x - 3 sin^2 x cos x

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