![image](https://tex.z-dn.net/?f=2+%5Ccos%5E%7B2%7D+x++-+7+%5Ccos+x+%3D+0+%5C%5C++%5Ccos+x+%3D+t+%5C%5C+2+%7Bt%7D%5E%7B2%7D+-+7t+%3D+0+%5C%5C+2t+-+7+%3D+0++%5C%5C+2t+%3D+7+%5C%5C++2%5Ccos+x+%3D+2+%5Ctimes+3.5+%3E+1+%5C%5Ct+%3D+0+%5C%5C++%5Ccos+x+%3D+0++%5C%5C+x+%3D++%5Cpm++%5Cfrac%7B%5Cpi%7D%7B2%7D++%2B+2%5Cpi+k+%5C%5C+)
1 \\t = 0 \\ \cos x = 0 \\ x = \pm \frac{\pi}{2} + 2\pi k \\ " alt="2 \cos^{2} x - 7 \cos x = 0 \\ \cos x = t \\ 2 {t}^{2} - 7t = 0 \\ 2t - 7 = 0 \\ 2t = 7 \\ 2\cos x = 2 \times 3.5 > 1 \\t = 0 \\ \cos x = 0 \\ x = \pm \frac{\pi}{2} + 2\pi k \\ " align="absmiddle" class="latex-formula">