1+tg^2a=1/cos^2a ctga=1/tga tg(a+b)=tga+tgb/1-tgatgb tg(a-b)=tga-tgb/1+tgatgb
ctga=1/tga
tg(a+b)=tga+tgb/1-tgatgb
tg(a-b)=tga-tgb/1+tgatgb