A(x1,y1),B(x2,y2),
AB的中点M（ (x1+x2)/2,(y1+y2)/2)
AB的斜率：k=(y2-y1)/(x2-x1)
AB的垂直平分线的斜率为:-1/k
因此由点斜式写出AB垂直平分线：y=-(x2-x1/(y2-y1)* [x-(x1+x2)/2]+(y1+y2)/2