x^2+y^2-2x+4y=0
(x-1)^2+(y+2)^2= 5
centre ( 1,-2)
P(2,1)截得弦长最长的直线方程
(y-1)/(x-2) = (-2-1)/(1-2)
= 3
y-1 = 3x -6
3x-y-5 = 0