y=kx+b
弦长d=√[(x1-x2)^2+(y1-y2)^2]
=√[(x1-x2)^2+k^2(x1-x2)^2]
=√（1+k^2）√[(x1-x2)^2]
=√（1+k^2）√[(x1+x2)^2- 4x1x2]
如果用y来表示
x=1/k(y-b)
就会得到d = √（1+k^2）[(x1+x2）^2 - 4x1x2] = √（1+1/k^2）[(y1+y2）^2 - 4y1y2]