Phương trình cos^2x + cos 2x - 3/4 = 0 có nghiệm khi

A. $x=\pm \frac{2\mathrm{\pi }}{3}+k\mathrm{\pi }, \mathrm{k}\in \mathrm{\mathbb{Z}}$

B. $x=\pm \frac{\mathrm{\pi }}{3}+k\mathrm{\pi }, \mathrm{k}\in \mathrm{\mathbb{Z}}$

C. $x=\pm \frac{\mathrm{\pi }}{6}+k\mathrm{\pi }, \mathrm{k}\in \mathrm{\mathbb{Z}}$

D. $x=\pm \frac{\mathrm{\pi }}{6}+k2\mathrm{\pi }, \mathrm{k}\in \mathrm{\mathbb{Z}}$