Kąt α jest ostry i sinα= 2/5. Wówczas cosα jest równy

$\displaystyle \frac{5}{2}$\displaystyle \frac{5}{2}

$\displaystyle \frac{\sqrt{21}}{4}$\displaystyle \frac{\sqrt{21}}{4}

$\displaystyle \frac{3}{5}$\displaystyle \frac{3}{5}

$\displaystyle \frac{\sqrt{21}}{5}$\displaystyle \frac{\sqrt{21}}{5}