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畫出這個圖,在面ACD上取CF中點M,連接ME,這樣ME與AF平行(中位線),題目就變成求ME與BE所成角的余弦值了,而正四面體每個面都是正三角形,所以三角形BME的每條邊都很好求,(有中點,就有直角,你可以設邊長為4)再用余弦定理即可!(課本上都有)
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