x^2+y^2-4mx+2my+20m-20=0x^2+y^2-20-4mx+2my+20m=0x^2+y^2-20-2m(2x-y-10)=0x^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。
