∫ arctanx dx= x * arctanx - ∫ x d(arctanx)= x * arctanx - ∫ x/(1+x²) dx= x * arctanx - (1/2)∫ d(x²)/(1+x²)= x * arctanx - (1/2)∫ d(1+x²)/(1+x²)= x * arctanx - (1/2)ln(1+x²) + C
