Найдите \(\displaystyle \cos\color{red}{\alpha}\small,\) если \(\displaystyle \sin\color{red}{\alpha}=\frac{2\sqrt{10}}{7}\) и \(\displaystyle 90º<\color{red}{\alpha}<180º\small.\)

\(\displaystyle \cos\color{red}{\alpha}=\)[ ]