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垂心. 設PH⊥平面ABC,H為垂足.由於直線AP,BP,CP兩兩垂直,所以AP⊥平面PBC 從而AP⊥BC,在底面ABC內,易知AH是PA的射影,由三垂線定理知,AH⊥BC, 同理,BH⊥AC,CH⊥AB,即H是垂心.
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