TRIG help quickly please

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Mark any solutions to the equation 2cos2x - 1 = 0. The answer can be more than one. Thank you


π/8
-7π/4
15π/4
3π/4

  • TRIG help quickly please -

    2 cos (2x) - 1 = 0
    cos (2x) = 1/2
    2x = π/3 or 2x = 2π-π/3 or 5π/3
    x = π/6 or x = 5π/6
    none of the given answers apply

    I will assume you meant
    2 cos^2 x - 1 = 0
    by definition:
    2cos^2 x -1 = cos (2x)

    so cos(2x) = 0
    2x = π/2 or 2x = 3π/2
    x = π/4 or x = 3π/4

    I see one of those answers. 3π/4

    looking at the others, note that -7π/4 is coterminal with π/4
    so it would be a solution as well

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