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