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方便點,設邊長為2,作三角形的CD邊的中線,中點F 中線延長到AB邊G,G是AB的中點 EF=√3,BE=2-√3,AE²;=BE²;=1+(2-√3)²; 余弦定理 cos∠AEB=(1+(2-√3)²;+1+(2-√3)²;-2²;)/2(1+(2-√3)²;)(1+(2-√3)²;) 化簡得cos∠AEB=-√3/2 也就是∠AEB為120°
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