用分步积分 ∫ln(x^2+1)dx =xln(x^2+1)-∫2x^2/(x^2+1)dx =xln(x^2+1)-∫(2x^2+2-2)/(x^2+1)dx =xln(x^2+1)-∫[2-2/(x^2+1)]dx =xln(x^2+1)-2x+2arctanx+C
