[tex]\displaystyle cos(\frac{x}{2}-\frac{\pi }{8})=0\\\\\frac{x}{2}-\frac{\pi }{8}=\frac{\pi }{2}+\pi n; n\in Z\\\\\frac{x}{2}=\frac{\pi }{2}+\frac{\pi }{8}+\pi n; n\in Z\\\\x=\frac{5\pi }{4}+2\pi n; n\in Z[/tex]