(1) To prove $\cos (\pi / 2-\theta)=\sin \theta$, we can use the sine and cosine addition formulas:
$\cos (\pi / 2-\theta) = \cos (\pi / 2) \cos (\theta) + \sin (\pi / 2) \sin (\theta)$
Since $\cos (\pi / 2) = 0$ and $\sin (\pi / 2) = 1$, this simplifies
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