ln(y)=(1/x)ln(x)
求导
（1/y）*y'=-(1/x^2)ln(x)+(1/x)*(1/x)=(1/x^2)[1-ln(x)]
y'=(1/x^2)*[1-ln(x)]*x^(1/x)=0
则1-ln(x)=0
x=e
极值为e^(1/e)