Ta có: \(sin(x+\frac{\pi}4)\le1\)
\(\Leftrightarrow 3sin(x+\frac{\pi}4)\le3\)
\(\Leftrightarrow 3sin(x+\frac{\pi}4)-7\le-4\) hay \(y\le-4\)
\(Max\ y=-4 \Leftrightarrow x+\frac{\pi}4=\frac{\pi}2+k2\pi\Leftrightarrow x=\frac{\pi}4+k2\pi\)
Chúc bạn học tốt!