Вопрос от посетителя
cos^4 a + cos^2 – sin^4 – sin^2 a=?
Отвечает Илюха:
cos^4 a + cos^2 – sin^4 – sin^2 a =
(cos^4 a – sin^4 a) + (cos^2 a – sin^2 a) =
(cos^2 a + sin^2 a)(cos^2 a – sin^2 a) + cos2a =
cos2a + cos2a = 2cos2a
cos^4 a + cos^2 – sin^4 – sin^2 a=?
cos^4 a + cos^2 – sin^4 – sin^2 a =
(cos^4 a – sin^4 a) + (cos^2 a – sin^2 a) =
(cos^2 a + sin^2 a)(cos^2 a – sin^2 a) + cos2a =
cos2a + cos2a = 2cos2a