三点A(x1,y1),B(x2,y2),C(x3,y3)共线的充要条件是(  ) A.x1y2-x2y1=0 B.x1y3-x3y1=0 C.(x2-x1)(y3-y1)=(x3-x1)(y2-y1) D.(x2-x1)(x3-x1)=(y

问题描述:

三点A(x1,y1),B(x2,y2),C(x3,y3)共线的充要条件是(  )
A. x1y2-x2y1=0
B. x1y3-x3y1=0
C. (x2-x1)(y3-y1)=(x3-x1)(y2-y1
D. (x2-x1)(x3-x1)=(y2-y1)(y3-y1

法一:若A,B,C三点共线则AB∥AC即(x2-x1,y2-y1)∥(x3-x1,y3-y1)则:(x2-x1)(y3-y1)=(x3-x1)(y2-y1)法二:若A,B,C三点共线则kAB=kAC即y2−y1x2−x1=y3−y1x3−x1即:(x2-x1)(y3-y1)=(x3-x1)...