Liên hợp nhé
$A=\dfrac{\bigg (\sqrt{1+\cos x}+\sqrt{1-\cos x} \bigg )^2}{1+\cos x -1+\cos x}=\dfrac{1+\cos x + 1-\cos x +2\sqrt{1-\cos^2 x}}{2\cos x}$
$=\dfrac{1+\sqrt{\sin^2 x}}{\cos x}=\dfrac{1-\sin x}{\cos x} =\dfrac{(\sin \dfrac{x}{2}-\cos \dfrac{x}{2} )^2}{\cos^2 \dfrac{x}{2} -\sin^2 \dfrac{x}{2}} =-\dfrac{\sin \dfrac{x}{2}- \cos \dfrac{x}{2}}{\sin \dfrac{x}{2}+ \cos \dfrac{x}{2}}$
$=-\dfrac{-\sqrt 2 \cos (\dfrac{x}{2}+\dfrac{\pi}{4})}{\sqrt 2 \sin (\dfrac{x}{2}+\dfrac{\pi}{4})}= XONG$