A(x1,y1),B(x2,y2),C(x3,y3)向量AB=(x2-x1,y2-y1),向量AC=(x3-x1,y3-y1)A、B、C共线得: 向量AB//向量AC(x2-x1)(y3-y1)=(x3-x1)(y2-y1)所以A、B、C共线:(x2-x1)(y3-y1)=(x3-x1)(y2-y1)
