点A在圆上,A(4,-3)在此切线上,切线的两点式方程为(y+3)/(x-4)=(y0+3)/(x0-4),① 圆心O,OA向量为(4,-3),切线向量为(x0-4,y0+3),OA垂直切线.则4(x0-4)-3(y0+3)=0,即(y0+3)/(x0-4)=4/3,代入①中,得到(y+3)/(x-4)=4/3,所...