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内(外)角平分线定理:如图1(图2),△ABC中,AD为∠BAC的内(外)角平分线的充要条件是(AB)/(AC)=(BD)/(DC).
The inner (external) angle bisector theorem: As shown in Fig. 1 (Fig. 2), in △ABC, the necessary and sufficient condition for the inner (outer) angle bisector of ∠BAC is (AB)/(AC) = (BD)/ (DC).