If m and n respectively are the numbers of positive and negative values of θ in the interval [-π, π] that satisfy the equation cos 2 θ cos (θ)/2= cos…

If $\displaystyle m$ and $\displaystyle n$ respectively are the numbers of positive and negative values of $\displaystyle \theta$ in the interval $\displaystyle [-\pi, \pi]$ that satisfy the equation $\displaystyle \cos 2 \theta \cos \frac{\theta}{2}=\cos 3 \theta \cos \frac{9 \theta}{2}$, then mn is equal to $\displaystyle \_\_\_\_$.