(1+ctg^2a)(cos^2a-1) Упростить
ctg x = cos x / sin x
sin² x + cos² x = 1
(1 + ctg² a)(cos² a - 1) = (1 + cos² a / sin² a) * (-sin² a) = (sin² a + cos² a) / sin² a *(-sin² a) = 1 *(-1) = -1