2^(-1/3)=1/2^(1/3)=1*2^(2/3)/2^(1/3+2/3)=2^(2/3)/2=(2^2)^(1/3)/2=4^(1/3)/2
4^(1/3)是4开立方
也可以这样：2^(-1/3)=(1/2)^(1/3)=0.5^(1/3)=0.79370052598409973737585281963615