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#### 口诀
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<font color='red' size=4><b>等线段，共端点，辅助线，用旋转</b></font>
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**解惑**：
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**$Q:$为什么要用旋转？**
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$A:$旋转后，由于等线段，可以完美边重合，可以构建全等三角形，使得问题转化为同一个三角形中的条件，问题就简化了。
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如图：
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延长$CB$至$E$，使得$BE=CD$
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$AC=AE,BE=CD,\angle ABE+\angle ABC=180^{\circ}$
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$\because \angle ABC+\angle ADC=180^{\circ}$
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$\therefore \angle ABE=\angle ADC$
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$\therefore \triangle ABE \cong ADC$
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$\therefore \angle 1=\angle 2,AC=AE$
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双 $\because \angle 1 +\angle 3=\angle 2+ \angle 3=90^{\circ}$
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$\therefore \triangle ACE$是等腰直角三角形
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而$S_{\triangle ACE}=$四边形面积
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$=\frac{1}{2}AC\cdot AE=18$
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$=\frac{1}{2} AC ^2=18$
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$\therefore AC=6$
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