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How to solve cos2x-cosx?

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How to solve cos2x-cosx?

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  1. 1. substitute the identity (cos 2x = 2cos^2 x - 1):
    --> cos2x - cosx = 2cos^2x - 1 - cosx

    2. rearrange the expression:
    --> cos2x - cosx = 2cos^2x - cosx - 1

    3. factorise the right side (2x^2 - x - 1):
    --> 2cosx = -1 and cosx = -1

    4. find the values of cos^-1(0.5) and cos^-1(-1)

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