\(\frac{cos\alpha}{1-\sin\alpha}=\frac{1+sin\alpha}{cos\alpha}\)
\(\Leftrightarrow cos^2\alpha=\left(1-sin\alpha\right)\left(1+\sin\alpha\right)\)
\(\Leftrightarrow cos^2\alpha=1-sin^2\alpha\)
\(\Leftrightarrow cos^2\alpha+sin^2\alpha=1\left(Đúng\right)\)