You have seen that the sine function equals 1 at $\displaystyle\theta=\frac{\pi}{{2}}\usepackage{cmbright}\Huge\$ . The graph of sine has a hill/valley pattern that repeats forever in both directions. Each hill and valley pattern together is $\displaystyle{2}\pi\usepackage{cmbright}\Huge\$ units wide. So the function will continue to be equal to 1 at $\displaystyle\frac{\pi}{{2}}\usepackage{cmbright}\Huge\$ past every multiple of $\displaystyle{2}\pi\usepackage{cmbright}\Huge\$ . All the multiples of $\displaystyle{2}\pi\usepackage{cmbright}\Huge\$ in the interval $\displaystyle{0}\le\theta\le{8}\pi\usepackage{cmbright}\Huge\$ are $\displaystyle\theta={0},{2}\pi,{4}\pi,{6}\pi,{8}\pi\usepackage{cmbright}\Huge\$ . So $\displaystyle{\sin{\theta}}={1}\usepackage{cmbright}\Huge\$ will occur at $\displaystyle\theta=\frac{\pi}{{2}}\usepackage{cmbright}\Huge\$ , $\displaystyle{2}\pi+\frac{\pi}{{2}}\usepackage{cmbright}\Huge\$ , $\displaystyle{4}\pi+\frac{\pi}{{2}}\usepackage{cmbright}\Huge\$ , $\displaystyle{6}\pi+\frac{\pi}{{2}}\usepackage{cmbright}\Huge\$ . Of course, these could also be written as $\displaystyle\theta=\frac{{1}}{{2}}\pi\usepackage{cmbright}\Huge\$ , $\displaystyle\frac{{5}}{{2}}\pi\usepackage{cmbright}\Huge\$ , $\displaystyle\frac{{9}}{{2}}\pi\usepackage{cmbright}\Huge\$ , $\displaystyle\frac{{13}}{{2}}\pi\usepackage{cmbright}\Huge\$ .