Cho \(\sin a + \cos a = \frac{5}{4}\). Khi đó \(\...

B

- Hướng dẫn giải

\(\begin{array}{l}
{\left( {\sin \alpha  + \cos \alpha } \right)^2} = {\sin ^2}\alpha  + 2\sin \alpha .\cos \alpha  + {\cos ^2}\alpha  = 1 + 2\sin \alpha .\cos \alpha \\
 \Rightarrow \sin \alpha .\cos \alpha  = \frac{{{{\left( {\sin \alpha  + \cos \alpha } \right)}^2} - 1}}{2} = \frac{9}{{32}}
\end{array}\)