Second order partial differential equation: $(x-y)^2/4 u_{xx} + (x-y) \sin(x^2+y^2) u_{xy} + \cos(x^2+y^2) u_{yy} +\dots=0$ is
a. Elliptic in $\{(x,y): x≠y, x^2+y^2<\frac{\pi}{6}\}$
b. Hyperbolic in $\{(x,y): x≠y, \frac{\pi}{4}
c. Elliptic in $\{(x,y): x≠y, \frac{\pi}{4}
d. Hyperbolic in $\{(x,y): x≠y, x^2+y^2<\frac{\pi}{4}\}$
I am stuck on this problem . Can anyone help me please......