tg(arctg(-2)+arctg 3)
tg(a+b)=(tg(a) + tg(b))/(1-tg(a)tg(b))
tg(arctg(-2))=-2
tg(arctg(3))=3
tg(arctg(-2) + arctg 3) = (-2+3)/(1+2*3)=1/7