x/(1+x)^4 =(x+1-1)/(1+x)^4 =1/(1+x)^3 -1/(1+x)^4 所以∫x/(1+x)^4 dx =∫1/(x+1)^3 dx -∫1/(x+1)^4 dx =-1/2 (x+1)^(-2) +1/3 (x+1)^(-3) +c
