Наталия Бельская
Оракул
(57581)
6 лет назад
(tg a - ctg a) \ (tg a + ctg a) =
= (sin a\cos a - cos a\sin a) \ ((sin a\cos a + cos a\sin a) =
= [(sin^2 a - cos^2 a)\(sin a * cos a)] \ [(sin^2 a + cos^2 a) \ sin a * cos a)] =
= [(sin^2 a - (1 - sin^2 a))\(sin a * cos a)] * (sin a * cos a)\1 =
= 2sin^2 a - 1