x^2+y^2-4mx+2my+20m-20=0
x^2+y^2-20-4mx+2my+20m=0
x^2+y^2-20-2m(2x-y-10)=0
x^2+y^2-20=0 (2x-y-10)=0 x=4,y=-2
则不论m取何值,当x=4,y=-2时,x^2+y^2-4mx+2my+20m-20=0恒成立.
x^2+y^2-4mx+2my+20m-20=0
x^2+y^2-20-4mx+2my+20m=0
x^2+y^2-20-2m(2x-y-10)=0
x^2+y^2-20=0 (2x-y-10)=0 x=4,y=-2
则不论m取何值,当x=4,y=-2时,x^2+y^2-4mx+2my+20m-20=0恒成立.