Tập nghiệm của phương trình 2 cos2x + 1 = 0 là. S = {pi/3

A. $S=\left\{\frac{\mathrm{\pi }}{3}+k2\mathrm{\pi }, -\frac{\mathrm{\pi }}{3}+\mathrm{k}2\mathrm{\pi },\mathrm{k}\in \mathrm{\mathbb{Z}}\right\}$

B. $S=\left\{\frac{2\mathrm{\pi }}{3}+k2\mathrm{\pi }, -\frac{2\mathrm{\pi }}{3}+\mathrm{k}2\mathrm{\pi },\mathrm{k}\in \mathrm{\mathbb{Z}}\right\}$

C. $S=\left\{\frac{\mathrm{\pi }}{3}+k\mathrm{\pi }, -\frac{\mathrm{\pi }}{3}+\mathrm{k\pi },\mathrm{k}\in \mathrm{\mathbb{Z}}\right\}$

D. $S=\left\{\frac{\mathrm{\pi }}{6}+k2\mathrm{\pi }, -\frac{\mathrm{\pi }}{6}+\mathrm{k}2\mathrm{\pi },\mathrm{k}\in \mathrm{\mathbb{Z}}\right\}$