积分x^3/(1+x^2)d(x)=积分[x*(x^2+1)-x]/(1+x^2)d(x) =积分x-x/(1+x^2)d(x) =积分xd(x)-积分x/(1+x^2)d(x) =1/2x^2-1/2积分1/(1+x^2)d(1+x^2) =1/2x^2-1/2ln(1+x^2)
