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