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)
