cho sina+cosa=5/4 Tính A=sina.cosz

$\displaystyle \begin{array}{{>{\displaystyle}l}} sina=cosa=\frac{5}{4}\\ \Longrightarrow ( sina+cosa)^{2} =\left(\frac{5}{4}\right)^{2}\\ \Leftrightarrow sin^{2} a+cos^{2} +2.sinacosa=\frac{25}{16}\\ \Longrightarrow \ 1+2A=\frac{25}{16}\\ \Longrightarrow \ A=\frac{\frac{25}{16} -1}{2} =\frac{9}{32} \end{array}$