2Cos²x - Cosx - 3 = 0
Сделаем замену :
Cosx = m, - 1 ≤ m ≤ 1
2m² - m - 3 = 0
D = (- 1)² - 4 * 2 * (- 3) = 1 + 24 = 25 = 5²
1\\\\Cosx=-1\\\\x=\pi+2\pi n,n\in z" alt="m_{1}=\frac{1-5}{4}=- 1\\\\m_{2}=\frac{1+5}{4}=1,5>1\\\\Cosx=-1\\\\x=\pi+2\pi n,n\in z" align="absmiddle" class="latex-formula">