∫x/(1+x^2)dx=1/2∫1/(1+x^2)dx^2=1/2∫1/(1+t)dt=1/2ln|1+t|+C=1/2ln(1+x^2)+C
∫x/(1+x^2)dx=1/2∫1/(1+x^2)d(1+x^2)=1/2ln(1+x^2)+C
∫x/(1+x²)dx=1/2∫2x/(1+x²)d(1+x²)=(1/2)ln(1+x²)+C