（1/3）∫(x^3)/(1+x^3)^2d(x^3)令t=x^3,则（1/3）∫t/(1+t)^2dt由部分积分法得,=(-1/3)t/(t+1)+(1/3)∫dt/(t+1)+c=(1/3)ln(t+1)-(1/3)t/(t+1)+c把t=x^3回代得，=(1/3)ln(x^3+1)-(1/3)x^3/(x^3+1)+c
