Obliczenia
[tex]tg\alpha=\frac{sin\alpha}{cos\alpha}\\\\\frac{sin\alpha}{cos\alpha}=2 \ \ /\cdot cos\alpha\\\\sin\alpha=2cos\alpha\\\\sin^2\alpha+cos^2\alpha=1\\\\(2cos\alpha)^2+cos^2\alpha=1\\\\4cos^2\alpha+cos^2\alpha=1\\\\5cos^2\alpha=1 \ \ /:5\\\\cos^2\alpha=\frac{1}{5}\\\\cos\alpha=\frac{1}{\sqrt5}\\\\\huge\boxed{cos\alpha=\frac{\sqrt5}{5}}[/tex]
