Kąt α jest ostry oraz sinα= 2√(5)/5. Wtedy

$\displaystyle \cos\alpha= \frac{5}{2\sqrt{5}}$\displaystyle \cos\alpha= \frac{5}{2\sqrt{5}}

$\displaystyle \cos\alpha= \frac{\sqrt{5}}{5}$\displaystyle \cos\alpha= \frac{\sqrt{5}}{5}

$\displaystyle \cos\alpha= \frac{1}{5}$\displaystyle \cos\alpha= \frac{1}{5}

$\displaystyle \cos\alpha= \frac{4}{5}$\displaystyle \cos\alpha= \frac{4}{5}