求导：f‘(x)=1/(1+x)-[2(x+2)+2x*1]/(x+2)²
=1/(x+1)-4(x+1)/(x+2)²
=[(x+2)²-4(x+1)²]/[(x+1)(x+2)²]
=-x(3x+4)/[(x+1)(x+2)²]