x^(-2)=y
x+y
=x^(-2)+x
=(1/2)x+(1/2)x+1/x^2
[平均值不等式]
>=3[(1/2)x*(1/2)x*1/x^2]^(1/3)
=3*(1/4)^(1/3)
=3/4^(1/3)
取等号时
1/2x=1/2x=1/x^2 x^3=1/2 x=1/2^(1/3)
请采纳.