Own work
cos ( θ ) = − 1 5 {\displaystyle \cos \left(\theta \right)=-{\frac {1}{\sqrt {5}}}\,}
cos ( θ ) = 1 5 {\displaystyle \cos \left(\theta \right)={\frac {1}{\sqrt {5}}}\,}
cos ( θ ) = ± 3 5 {\displaystyle \cos \left(\theta \right)=\pm {\sqrt {\frac {3}{5}}}\,}