[tex]\frac{1}{tg^2\alpha}+1=\frac{1}{sin^2\alpha}\\\\L=\frac{1}{tg^2\alpha}+1=ctg^2\alpha+1=\frac{cos^2\alpha}{sin^2\alpha}+1=\frac{cos^2\alpha}{sin^2\alpha}+\frac{sin^2\alpha}{sin^2\alpha}=\\\\\frac{cos^2\alpha+sin^2\alpha}{sin^2\alpha}=\frac{1}{sin^2\alpha}=P[/tex]
