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$$\sin (\theta + 90^\circ) = \cos\theta,\hspace{1zw} \cos (\theta + 90^\circ) = -\sin\theta$$ (3)

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$$\mbox{$\sin x$, $\cos x$\ $B$K4X$9$k0l<!<0(B\ ($B$b$A$m$s(B $\sin y$, $\cos y$\ $B$K4X$7$F$b(B)}$$ (4)

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$$\sin(x+y) = A\sin x + B\cos x,\hspace{1zw} \cos(x+y) = C\sin x + D\cos x$$ (5)

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$$\sin y = A\sin 0^\circ + B\cos 0^\circ = B,\hspace{1zw} \cos y = C\sin 0^\circ + D\cos 0^\circ = D$$

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$$\begin{eqnarray*} \sin (90^\circ + y) &= & \cos y= A\sin 90^\circ + B\cos 90^\c... ...(90^\circ + y) &=& -\sin y = C\sin 90^\circ + D\cos 90^\circ = C \end{eqnarray*}$$

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