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$B:#(B $AD=1$ $B$H$9$k$H(B $AC=\cos y$ $B$H$J$k$N$G(B $AB = AC\cos x=\cos y\cos x$ $B$H$J$k!#(B

$B0lJ}(B $CD= \sin y$ $B$G$"$j!";03Q7A(B $AEF$ $B$H;03Q7A(B $DCF$ $B$OAj;w$J$N$G(B $B3Q(B $CDF=x$ $B$G$"$j!"$h$C$F(B $BE = CG = CD\sin x = \sin x\sin y$ $B$H$J$k!#(B $B$f$($K(B

$$\cos(x + y) = AE = AB-BE = \cos x\cos y - \sin x\sin y$$

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$B$^$?!"(B $DG = CD\cos x = \sin y \cos x$, $GE = CB = AC\sin x = \cos y \sin x$ $B$H$J$k$N$G!"(B

$$\sin(x + y) = DE = GE + DG = \sin x\cos y + \cos x\sin y$$

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$B$3$N9b$5(B $AD=1$ $B$H$9$k$H!"(B

$$AB = \frac{1}{\cos x},\hspace{1zw} BD = AB\sin x = \frac{\sin x}{\cos x} =\tan x$$

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$$AC = \frac{1}{\cos y},\hspace{1zw} CD = AC\sin y = \frac{\sin y}{\cos y} =\tan y$$

$B$h$j(B $BC = BD + CD = \tan x + \tan y$ $B$G$"$k!#(B $B$h$C$F!"M>89DjM}(B

$$BC^2 = AB^2 + AC^2 -2AB\cdot AC\cos (x+y)$$

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$$(\tan x + \tan y)^2 = \frac{1}{\cos^2 x} + \frac{1}{\cos^2 y} -2\frac{1}{\cos x}\cdot\frac{1}{\cos y}\cos(x+y)$$

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$$\begin{eqnarray*} \lefteqn{2\frac{1}{\cos x}\cdot\frac{1}{\cos y}\cos(x+y)}\\ ... ... y}\\ & = & 2\frac{\cos x\cos y - \sin x\sin y}{\cos x\cos y} \end{eqnarray*}$$

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$$\triangle ABD = \frac{1}{2}AD\cdot BD = \frac{1}{2}\tan x, ... ...w} \triangle ACD = \frac{1}{2}AD\cdot CD = \frac{1}{2}\tan y$$

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$$\triangle ABC = \frac{1}{2}AB\cdot AC\sin(x+y) = \frac{\sin(x+y)}{2\cos x\cos y}$$

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$$\frac{\sin(x+y)}{\cos x\cos y} = \tan x+\tan y = \frac{\sin x}{\cos x} + \frac{\sin y}{\cos y}$$

$B$H$J$j!"N>JU$K(B $\cos x\cos y$ $B$r$+$1$l$P(B sin $B$N2CK!DjM}(B (1) $B$,F@$i$l$k$3$H$K$J$k!#(B

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$$\left[ \begin{array}{rr} \cos(x+y) & -\sin(x+y)\\ \sin(x+... ...}{rr} \cos y & -\sin y\\ \sin y & \cos y \end{array}\right]$$

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