\tan\alpha * \frac{\cot\alpha}{1-\sin^2\alpha} = \tan^2\alpha+1
L=\tan\alpha * \frac{\ctg\alpha}{1-\sin^2\alpha}=\frac{\sin\alpha}{\cos\alpha}*\frac{\frac{\cos\alpha}{\sin\alpha}}{cos^2\alpha}=\frac{1}{\cos^2\alpha}=\frac{\sin^2\alpha+\cos^2\alpha}{\cos^2\alpha}=\frac{\sin^2\alpha}{\cos^2\alpha}+\frac{\cos^2\alpha}{\cos^2\alpha}=\tan^2\alpha+1=P