x^2+y^2-2dx+4cy-4=0
(x-d)^2+(y+2c)^2=d^2+4c^2+4
圆心：C(d,-2c)
圆心在x-y+1=0上
d+2c+1=0
-2c=d+1
C(d,d+1),(x-d)^2+(y-d-1)^2=2d^2+2d+5
圆C经过点(1,5)
(1-d)^2+(4-d)^2=2d^2+2d+5
d=1
圆C：(x-1)^2+(y-2)^2=9
动直线l:y=-x+m与圆C交於A,B两点
(x-1)^2+(-x+m-2)^2=9
2x^2+(2-2m)x+m^2-4m-4=0
AB中点：xp=(x1+x2)/2=(m-1)/2,yp=(y1+y2)/2=(-x1+m-x2+m)/2=(m+1)/2
|AB|=√[(x1-x2)^2+(y1-y2)^2]
=√[(x1-x2)^2+(-x1+m+x2-m)^2]
=√[2(x1-x2)^2]
=√2*√[(x1+x2)^2-4x1x2]
=√2*√[(m-1)^2-2m^2+8m+8]
=√2*√(-m^2+6m+9)
所以圆P：[x-(m-1)/2]^2+[y-(m+1)/2]^2=(-m^2+6m+9)/2
假设圆P过原点,那么(m-1)^2/4+(m+1)^2/4=(-m^2+6m+9)/2
m=4 或者 m=-1
m=4时
圆P：(x-3/2)^2+(y-5/2)^2=17/2
L：y=-x+4
m=-1时
圆P：(x+1)^2+y^2=1
L：y=-x-1