∫ x³/(3 + x²) dx= ∫ x[(3 + x²) - 3]/(3 + x²) dx= ∫ x dx - 3∫ x/(3 + x²) dx= x²/2 - (3/2)∫ d(3 + x²)/(3 + x²)= x²/2 - (3/2)ln(3 + x²) + C
