chứng minh (1-sin2a)/(1+sin2a)=cot^2(pi/4+a)

$VP=\dfrac{\cos^2\Big(\dfrac{\pi}{4}+a\Big) }{\sin^2\Big(\dfrac{\pi}{4}+a\Big)}$

$=\dfrac{1+\cos\Big(\dfrac{\pi}{2}+2a\Big) }{1-\cos\Big(\dfrac{\pi}{2}+2a\Big) }$ 

$=\dfrac{1-\sin2a}{1+\sin2a}$

$=VT$