--------------------------------------------------------

--------------------------------------------------------

here x and y are the independent variables
when differentitating partially w.r.t.x, treat y as a constant
when differentitating partially w.r.t.y, treat x as a constant
for ∂²u/∂x² diff. ∂u/∂x again w.r.t.x partially
for ∂²u/∂x∂y diff.∂u/∂x partially again w.r.t.y partially
  answer and  some steps are given below

================
* *
================