а) (1-ctg y)/(tg y - 1) = -(ctg y - 1)/(tg y - 1) = -1
b) tg^2a(sin^2a - 1) = tg^2a(-cos^2a) = -tg^2a*cos^2a
а) (1-ctg y)/(tg y - 1) = -(ctg y - 1)/(tg y - 1) = -1
b) tg^2a(sin^2a - 1) = tg^2a(-cos^2a) = -tg^2a*cos^2a