Answer
$\frac{7\pi}{6}, \frac{11\pi}{6}$
Work Step by Step
$2sin\theta+1=0 \longrightarrow sin\theta=-\frac{1}{2} \longrightarrow \theta=\pi+\frac{\pi}{6}, 2\pi-\frac{\pi}{6} \longrightarrow \theta=\frac{7\pi}{6}, \frac{11\pi}{6}$
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