identities trig?

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find all solutions to the equation in the interval [0,2pie]

cos2x=cosx

  • identities trig? -

    See:
    http://www.jiskha.com/display.cgi?id=1303866873

  • identities trig? -

    cos 2x = cosx
    2cos^2 x - 1 - cosx = 0
    (2cosx + 1)(cosx -1) = 0
    cosx = -1/2 or cosx = 1

    if cosx = -1/2, x is in II or III
    x = 2π/3 or 4π/3 ......I knew cos π/3 = +1/2

    if cosx = 1, then x = 0 or x = 2π

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