原式=∫(x-9x/(9+x^2))dx=∫xdx-∫9x/(9+x^2)dx=x^2/2-9ln(x^2+9)/2+C
∫x^3/(9+x^2)dx=1/2∫x^2/(9+x^2)dx^2 (x^2=t)=1/2∫t/(9+t)dt=1/2∫(t+9-9)/(9+t)dt=1/2∫[1-9/(9+t)]dt=1/2t-9/2ln(9+t)+C=1/2x^2-9/2ln(9+x^2)+C
