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a^2+b^2-2ab cos A=l1^2 a^2+d^2-2ad cos(180-A)=l2^2 c^2+b^2-2cb cos(180-A)=l2^2 c^2+d^2-2cd cos A=l1^2 四式相加就 由於cos A + cos(180-A)=0可得到答案了
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