y=x+m
x^2+y^2=2
将y=x+m代入x^2+y^2=2
x^2+(x+m)^2=2
2x^2+2mx+(m^2-2)=0
有两个相等实数解
判别式 为
（2m)^2-8(m^2-2)
=4m^2-8m^2+16
=16-4m^2
=0
得m=+(-)2