Mark M. answered 04/28/16
Tutor
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Retired Math prof with teaching and tutoring experience in trig.
sin3x - sinx = 0
sinx(sin2x - 1) = 0
sinx(sinx - 1)(sinx + 1) = 0
sinx = 0, sinx = 1, or sinx = -1
Solutions in the interval [0, 2π] are 0, π/2, π, 3π/2, 2π