Cho góc α thỏa mãn sin⁡α + cos⁡α = \(\frac{{\sqrt...

B

- Hướng dẫn giải

Ta có 

\(\begin{array}{l}
{\rm{co}}{{\rm{s}}^2}\alpha  = 1 - {\sin ^2}\alpha  = 1 - {\left( {\frac{{2\sqrt 6 }}{5}} \right)^2} = \frac{1}{{25}}\\
{90^ \circ } < \alpha  < {180^ \circ } \Rightarrow {\rm{cos}}\alpha {\rm{ <  0}}\\
{\rm{co}}{{\rm{s}}^2}\alpha  = \frac{1}{{25}},{\rm{cos}}\alpha  < 0 \Rightarrow {\rm{cos}}\alpha  =  - \frac{1}{5}
\end{array}\)