[tex]cos \theta = \frac{ - 2 \sqrt{5} }{5} [/tex]
[tex]cos 2\theta = 2cos {}^{2} \theta - 1 \\ cos2 \theta = 2( \frac{ - 2 \sqrt{5} }{5} ) {}^{2} - 1 \\ cos2 \theta = 2( \frac{20}{ \\ 25} ) - 1 \\ cos2 \theta = \frac{40 - 25}{25} = \frac{15}{25} = \frac{3}{5} [/tex]
