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我们知道,在直角坐标系中,设点P_1(x_1,y_1)、P_2(x_2,y_2),若点P(x,y)为有向线段P_1P_2的内(外)分点,则点P分P_1P_2所成的比λ为 λ=(P_1P)/(PP_2)=(x-x_1)/(x_2-x)(=(y-y_1)/(y_2-y)>0(<0)。 (*) 特别地,当线段P_1P_2落在x轴上时,纵坐标为0,情形就更加明了(以下讨论仅在x轴上进行,且不妨约定x_10(λ<0),则P为P_1P_2的内(外)分点,亦即P点介于P_1P_2之间(之外),这时有x_10(<0).(* In particular, when the segment P_1P_2 falls on the x-axis and the ordinate is 0, the situation becomes clearer (the following discussion is only performed on the x-axis, and x_1 0 (λ< 0), then P is the inner (outer) point of P_1P_2, that is, P point is between P_1P_2 (outside), at this time x_1
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