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    Prove the identity

    cos 4x + 4cos2x = 8cos^4x - 3.

    Trust me, I have attempted this question, but I cant post the attempt as it is on a piece of paper and is too bloody long.

    I would hope someone would be kind enough to post the step-by-step solution.
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    (Original post by Ari Ben Canaan)
    Prove the identity

    cos 4x + 4cos2x = 8cos^4x - 3.

    Trust me, I have attempted this question, but I cant post the attempt as it is on a piece of paper and is too bloody long.

    I would hope someone would be kind enough to post the step-by-step solution.
    It comes out in 3 lines.

    Start with the LHS and and repeatedly use the identity \cos2Y= 2\cos^2Y-1

    Where Y will sometimes be 2x, and sometimes x, depending on what you're expanding.
 
 
 
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