You want to find values of [tex]\theta[/tex] where that equation is true.
You can do this by solving the equation.
First:
[tex]-1-2 \cos{\theta} = -1[/tex]
[tex]-2 \cos{\theta} = 0[/tex]
[tex]\cos{\theta} = 0[/tex]
For what values of [tex]\theta[/tex] is the cosine function 0? Hint: look at the graph of cosine
