Tg a = 2/5 tg b = 1/3 вычислите tg (a+b)

Tg (a+b) = (tga+tgb) / (1-tgatgb) = (1/3-2) / (1-1/3 * (-2)) = (-5/3) / (1+2/3) = (-5/3) / (5/3) = - 1

ctg (a-b) = 1/tg (a-b) = (1+tgatgb) / (tga-tgb) = (1+1/3 * (-2)) / (1/3+2) = (1-2/3) / (7/3) = (1/3) / (7/3) = 1/7