$$\cos \theta = \frac { 3 } { 5 }$$
$\theta =2\pi n_{1}+\arctan(\frac{4}{3})\text{, }n_{1}\in \mathrm{Z}$
$\theta =2\pi n_{2}+2\pi -\arctan(\frac{4}{3})\text{, }n_{2}\in \mathrm{Z}$
$\theta =2\pi n_{1}+\arccos(\frac{3}{5})\text{, }n_{1}\in \mathrm{Z}$
$\theta =2\pi n_{2}+2\pi -\arccos(\frac{3}{5})\text{, }n_{2}\in \mathrm{Z}$
