Question

Срочно помогите решить

Answer 1

$2 cos^{2} x - 7 cos x = 0 \ cos x = t \ 2 {t}^{2} - 7t = 0 \ 2t - 7 = 0 \ 2t = 7 \ 2cos x = 2 times 3.5 > 1 \t = 0 \ cos x = 0 \ x = pm frac{pi}{2} + 2pi k$