Прочее

Преобразуем уравнение: $$6(1 - \sin^2 x) + 5\sqrt{2}\sin x + 2 = 0$$ $$-6\sin^2 x + 5\sqrt{2}\sin x + 8 = 0$$ $$6\sin^2 x - 5\sqrt{2}\sin x - 8 = 0$$

Сделаем замену: $t = \sin x \in [-1; 1]$: $$6t^2 - 5\sqrt{2}t - 8 = 0$$ $$D = 50 + 4\cdot8\cdot6 = 242$$ $$t = \frac{5\sqrt{2} \pm \sqrt{242}}{12}$$ $$t = \frac{5\sqrt{2} \pm 11\sqrt{2}}{12}$$ $$t = \frac{4\sqrt{2}}{3} \quad \notin [-1; 1] \quad \text{или} \quad t = -\frac{\sqrt{2}}{2}$$ Обратная замена: $$\sin x = -\frac{\sqrt2}{2}$$ $$x = -\frac{\pi}{4} + 2\pi k \quad \text{или} \quad x = -\frac{3\pi}{4} + 2\pi k, k \in Z$$

Используем формулу для $\cos 2x$: $$\cos 2x = 1 - 2\sin^2 x$$ Подставим в уравнение: $$3(1 - 2\sin^2 x) - 5\sin x + 1 = 0$$ $$3 - 6\sin^2 x - 5\sin x + 1 = 0$$ $$-6\sin^2 x - 5\sin x + 4 = 0$$ $$6\sin^2 x + 5\sin x - 4 = 0$$ Сделаем замену $t = \sin x \in [-1; 1]$: $$6t^2 + 5t - 4 = 0$$ $$D = 25 + 96 = 121$$ $$t = \frac{-5 \pm 11}{12}$$ $$t = \frac{1}{2} \quad \text{или} \quad t = -\frac{4}{3} \notin [-1; 1]$$ $$\sin x = \frac{1}{2}$$ $$x = \frac{\pi}{6} + 2\pi k, \quad x = \frac{5\pi}{6} + 2\pi k, \quad k \in \mathbb{Z}$$

Выразим $\sin^2x$ через $\cos x$: $$2\cos x + 1 - \cos^2x = 2\cos^3x$$ $$2\cos^3x + \cos^2x - 2\cos x - 1 = 0$$ Сделаем замену $t = \cos x$: $$2t^3 + t^2 - 2t - 1 = 0 \Leftrightarrow 2t(t^2 - 1) + t^2 - 1 = 0 \Leftrightarrow$$ $$\Leftrightarrow (t - 1)(t + 1)(2t + 1) = 0$$ $$t = -1 \quad \text{или} \quad t = 1 \quad \text{или} \quad t = -\frac{1}{2}$$ $$\cos x = -1 \quad \text{или} \quad \cos x = 1 \quad \text{или} \quad \cos x = -\frac{1}{2}$$ $$x = \pi k, \quad x = \pm\frac{2\pi}{3} + 2\pi n, \quad k, n, \in \mathbb{Z}$$

$\left(*\right) \cos x \neq 0 \Leftrightarrow x \neq \frac{\pi}{2} + \pi k, k \in Z;$

Сделаем замену $t = \tg x$: $$t^3 + t^2 - 3t - 3 = 0$$ $$(t^3 + t^2) - (3t + 3) = 0$$ $$t^2(t + 1) - 3(t + 1) = 0$$ $$(t + 1)(t^2 - 3) = 0$$ $$t = -1 \text{ или } t^2 = 3$$ $$\tg x = -1 \Rightarrow x = -\frac{\pi}{4} + \pi k, \quad k \in \mathbb{Z}$$ $$\tg x = \pm \sqrt{3} \Rightarrow x = \pm \frac{\pi}{3} + \pi k, \quad k \in \mathbb{Z}$$