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$$\sin (\theta + 90^\circ) = \cos\theta,\hspace{1zw} \cos (\theta + 90^\circ) = -\sin\theta$$ (7)

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$$\begin{eqnarray*} \mbox{$B:8JU(B} & = & \sin(x+90^\circ+y) = \cos(x+y),\\ \mbox{.. ...s y+\cos(x+90^\circ)\sin y\\ & = & \cos x\cos y - \sin x\sin y \end{eqnarray*}$$

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$$\begin{array}{ll} \sin (90^\circ - x) = \cos x,& \cos (90... ...in x,\\ \sin(-x) = -\sin x, & \cos(-x) = \cos x \end{array}$$ (8)

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$$\sin (90^\circ+\theta) = \cos(-\theta) = \cos\theta,\hspace{1zw} \cos (90^\circ+\theta) = \sin(-\theta) = -\sin\theta$$

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