[tex]cos(4\theta) - cos(2\theta) = sin(\theta)[/tex]
[tex]-2sin(\frac{4\theta + 2\theta}{2})sin(\frac{4\theta - 2\theta}{2}) = sin(\theta)[/tex]
[tex]-2sin(\frac{6\theta}{2}sin\frac{2\theta}{2}) = sin(\theta)[/tex]
[tex]-2sin(3\theta)sin(\theta) = sin(\theta)[/tex]
[tex]\frac{-2sin(3\theta)sin(\theta)}{-2sin(\theta)} = \frac{sin(\theta)}{-2sin(\theta)}[/tex]
[tex]sin(3\theta) = -\frac{1}{2}[/tex]
[tex]sin^{-1}[sin(3\theta)] = sin^{-1}(-\frac{1}{2})[/tex]
[tex]3\theta = -30[/tex]
[tex]\frac{3\theta}{3} = \frac{-30}{3}[/tex]
[tex]\theta = -10[/tex]
