Bình phương hai vế ta có:
\((\sin a+\cos a)^2=\dfrac{16}{9}\)
\(\Rightarrow {\sin}^2a+2\sin a\cos a+{\cos}^2a=\dfrac{16}{9}\)
\(\Rightarrow {\sin}^2a+{\cos}^2a+2\sin a\cos a=\dfrac{16}{9}\)
\(\Rightarrow 1+2\sin a\cos a=\dfrac{16}{9}\)
\(\Rightarrow M=\sin x\cos x=\dfrac{7}{18}\).