10. Ta có: $\frac{\pi}{12}+\frac{11\pi}{12}=\frac{3\pi}{12}+\frac{9\pi}{12}=\frac{5\pi}{12}+\frac{9\pi}{12}=\pi$$\Rightarrow cos^2\frac{11\pi}{12}=cos^2\frac{\pi}{12};....$ Tương tự
Viết lại $A=cos^2\frac{\pi}{12}+cos^2\frac{3\pi}{12}+cos^2\frac{5\pi}{12}+cos^2\frac{7\pi}{12}+cos^2\frac{9\pi}{12}$
$=2(cos^2\frac{\pi}{12}+cos^2\frac{\pi}{4}+cos^2\frac{5\pi}{12})$
Ta có: $cos^2\frac{5\pi}{12}=sin^2\frac{\pi}{12}$
$\Rightarrow A=2(1+\frac{1}{2})=3$