Answer
$\frac{3\pi}{4},\frac{7\pi}{4}$
Work Step by Step
Using trigonometric identities, we have
$cos(2\theta-\frac{\pi}{2})=-1 \longrightarrow 2\theta-\frac{\pi}{2}=\pi, 3\pi \longrightarrow \theta=\frac{3\pi}{4},\frac{7\pi}{4}$
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