Partial derivatives on area of a triangle
Solution 1:
You won't find an expression for $\omega(x,y)$ because it doesn't exist. The angle between two sides is free to vary when only their lengths are specified.
Generally a triangle cannot be specified (up to an isometry) without three parameters (SAS, SSS work, for example).
I hope writing $E = E(x,y,\omega)$ will then make your problem simple.