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