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[几何] 立体几何

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guanmo1 Post time 2015-11-2 22:41 |Read mode

立体几何.jpg

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kuing Post time 2015-11-2 22:49

作 AB 上的高 CH，只要 $\angle BCD\geqslant \angle HCD$ 即可，下略

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Author| guanmo1 Post time 2015-11-3 16:27

回复 2# kuing

简洁有力！

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敬畏数学 Post time 2015-11-4 09:25

取BC中点E，只需ADE为等腰三角形，或者BC在ACD所在平面上，构成ABDC棱长为1锐角为60°的棱形。答案A。
此题选择题，直接否定BCD。不可能大于等于2.

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