[tex]cos\alpha=\frac{1}{1+tg^2\alpha}\\\\P=\frac{1}{1+tg^2\alpha}=\frac{1}{1+\frac{sin^2\alpha}{cos^2\alpha}}=\frac{1}{\frac{cos^2\alpha}{cos^2\alpha}+\frac{sin^2\alpha}{cos^2\alpha}}=\frac{1}{\frac{cos^2\alpha+sin^2\alpha}{cos^2\alpha}}=\frac{1}{\frac{1}{cos^2\alpha}}=cos^2\alpha\neq P[/tex]
To nie jest tożsamość.
