How do I transform the following PDE into canonical form given trigonometric coefficients? I have tried the usual method using the quadratic formula but I get nowhere. Please help!
$$\cos(\theta)u_{xx}+2\sin(\theta)u_{xy}-\cos(\theta)u_{yy}$$
Note that $\theta$ does not have any restriction on it. Note comments below. We classify it as a hyperbolic PDE. I'm having trouble putting into the canonical after the you use the common technique.