-2/3(1十x)的导数=-2/3*(1十x)的导数=-2/3-2/[3(1十x)]的导数=2/[3*(1十x)]²*3=2/3*1/(1十x)²
[(-2/3)(1+x)]' = -2/3{(-2)/[3(1+x)]}' = (-2/3)[1/(1+x)]' = (2/3)/(1+x)^2
