2. y_{n+1} = (n + 1)^2 - 5 * (n + 1) + 6 = n^2 - 3n + 2
y_n = n^2 - 5n + 6
Получаем неравенство n^2 - 3n + 2 > n^2 - 5n + 6
Отсюда 2n > 4 => n > 2
4. 1/16 + cos^4 x - 1/2 * cos^2 x = (1/4 - cos^2 x)^2
9/16 + cos^4 x - 3/2 * cos^2 x = (3/4 - cos^2 x)^2
6. Метод интервалов.