The answer would be
#pi/2; (7pi)/6 and (11pi)/6#
Explanation ?
Call sin x = t.
#2t^2 - t - 1 = 0.#
Since a + b + c = 0. use shortcut. 2 real roots: t = 1 and #t = c/a = -1/2#
sin x = t = 1 --> #x = pi/2#
sin x = t = -1/2 --> #x = (7pi)/6# and #x = (11pi)/6#
Within interval #(0, 2pi)#: 3 answers: #pi/2; (7pi)/6 , and (11pi)/6.#
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