\(sin\left(x+\frac{\pi}{4}\right)=1\Rightarrow x+\frac{\pi}{4}=\frac{\pi}{2}+k2\pi\Rightarrow x=\frac{\pi}{4}+k2\pi\)
\(\Rightarrow\pi\le\frac{\pi}{4}+k2\pi\le5\pi\Rightarrow\frac{3}{8}\le k\le\frac{19}{8}\)
\(\Rightarrow k=\left\{1;2\right\}\Rightarrow\) có 2 nghiệm