## Trigonometry (11th Edition) Clone

The solution set is: $\{90^{\circ}+180^{\circ}n, ~60^{\circ}+360^{\circ}n, ~300^{\circ}+360^{\circ}n\}$
$cos~\theta-1= cos~2\theta$ $cos~\theta-1= 2~cos^2\theta-1$ $cos~\theta= 2~cos^2\theta$ $cos~\theta = 0~~$ or $~~cos~\theta = \frac{1}{2}$ $\theta = 90^{\circ}+180^{\circ}n$ $\theta = 60^{\circ}+360^{\circ}n$ $\theta = 300^{\circ}+360^{\circ}n$ The solution set is: $\{90^{\circ}+180^{\circ}n, ~60^{\circ}+360^{\circ}n, ~300^{\circ}+360^{\circ}n\}$