Giải thích các bước giải:
$VP=\cot^2\left ( \dfrac{\pi}{4}-\dfrac{\alpha}{2} \right )\\
=\dfrac{1}{\sin^2\left ( \dfrac{\pi}{4}-\dfrac{\alpha}{2} \right )}-1\\
=\dfrac{2}{1-\cos\left ( 2.\dfrac{\pi}{4}-2.\dfrac{\alpha}{2} \right )}-1\\
=\dfrac{2}{1-\cos\left ( \dfrac{\pi}{2}-\alpha \right )}-1\\
=\dfrac{2}{1-\sin\alpha}-1\\
=\dfrac{2}{1-\sin\alpha}-\dfrac{1-\sin\alpha}{1-\sin\alpha}\\
=\dfrac{2-1+\sin\alpha}{1-\sin\alpha}\\
=\dfrac{1+\sin\alpha}{1-\sin\alpha}=VT\Rightarrow đpcm$
