(1)P1,P2,P三点共线,用定比分点公式Px=x1+kx2/1+kPy=y1+ky2/1+kp(x1+kx2/1+k,y1+ky2/1+k)(2)P(x1+x2/2,y1+y2/2) 此时P为P1P2中点