[tex]sin\alpha+sin\alpha\cdot tg^2\alpha=\frac{tg\alpha}{cos\alpha}[/tex]
[tex]L=sin\alpha+sin\alpha\cdot tg^2\alpha=sin\alpha+sin\alpha\cdot tg\alpha\cdot tg\alpha=[/tex]
[tex]sin\alpha+sin\alpha\cdot \frac{sin^2\alpha}{cos^2\alpha}=sin\alpha+\frac{sin^3\alpha}{cos^2\alpha}=[/tex]
[tex]sin\alpha+\frac{sin^3\alpha}{cos^2\alpha}= \frac{sin\alpha\cdot cos^2\alpha}{cos^2\alpha}+\frac{sin^3\alpha}{cos^2\alpha}=[/tex]
[tex]\frac{sin\alpha\cdot cos^2\alpha+sin^3\alpha}{cos^2\alpha}=\frac{sin\alpha(cos^2\alpha+sin^2\alpha)}{cos^2\alpha}=\frac{sin\alpha\cdot 1}{cos^2\alpha}=[/tex]
[tex]\frac{sin\alpha}{cos^2\alpha}=\frac{\frac{sin\alpha}{cos\alpha}}{cos\alpha}=\frac{tg\alpha}{cos\alpha}=P[/tex]
