Con2a+cos14a+cos6a+cos10a Пожалуйста

Cos2a + cos14a + cos6a + cos10a = 2 * cos((2*a + 14*a) / 2) * cos((2*a - 14*a) / 2) + 2 * cos((6*a + 10*a) / 2) * cos((6*a - 10*a) / 2) = 2 * cos8a * cos6a + 2 * cos8a * cos2a = 2 * cos8a * (cos6a + cos2a )= 2 * cos8a * 2 * cos4a * cos2a = 4*cos8a * cos4a * cos2a.