因为 (x³ - 1) = (x³ + x ) - ( x + 1)＝x(x² + 1 ) - ( x + 1)
I＝∫(x^3-1)/(x^2+1)dx
＝∫xdx - ∫(x + 1)/(x² + 1)dx
＝∫xdx - ∫x /(x² + 1)dx - ∫1 /(x² + 1)dx
＝x²/2 - 1/2 * ln|(x² + 1)| - arctanx + C