I'm trying to solve $\displaystyle a\left(\cos^{2}{\theta} - \sin^{2}{\theta}\right) + h\left(\cos{\theta} - \sin{\theta}\right) = 0$ for $\displaystyle \theta$ where $\displaystyle 0 \le \theta \le \frac{\pi}{2}$, but I can't see the best approach. Any thoughts?