（x2）+（y2）-2x+4my+3（m2）=0
(x-1)^2+(y+2m)^2=4m^2+1-3m^2=m^2+1
圆心坐标是（1,-2m),在直线x+y+2=0上,则有：
1-2m+2=0
得m=3/2
半径=根号（m^2+1)=根号（9/4+1）=根号13 /2