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How does cos(x)+2cos^2 (x) -1=0 become cos(x)=1/2 or -1?

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  1. You are looking at a quadratic with cosx as the variable
    we could do this:
    let k = cosx, then your equation becomes
    k + 2k^2 - 1 = 0
    2k^2 + x - 1 = 0
    (2k - 1)(k + 1) = 0
    k = 1/2 or k = -1
    then cosx = 1/2, cosx = -1

    your next step probably would be to solve it for x

  2. Thank man. I would never get it without your help.

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