∫ arcctgx dx=[u=arcctgx, du=-dx/(1+x^2), dv=dx, v=x]=x*arcctgx-∫(-x/(1-x^2))*dx=x*arcctgx+1/2∫(d(1+x^2))/(1+x^2)=x*arcctgx+(1/2)*ln(1+x^2)+c
