(x^2+2y^2)
=(x^2+2y^2)*(1/x^2+1/y^2)
>=(x*(1/x)+根号2*y*(1/y))^2
=(1+根号2)^2
=3+2根号2
等号在x=根号2*y时取到
因此最小值是3+2根号2