The degree of a differential equation is the highest power of the highest order derivative when the equation is in polynomial form (no roots, no fractions with derivatives).
Given equation:
(2+(dxdy)2)23=a2dx2d2y
The highest order derivative in the equation is dx2d2y (second order).
The left side has a fractional power 23, so the equation is not in polynomial form.
Squaring both sides:
(2+(dxdy)2)232=[a2dx2d2y]2
(2+(dxdy)2)3=a4(dx2d2y)2
The equation is now in polynomial form.
The highest order derivative dx2d2y appears with power 2.
Therefore, the degree of the differential equation is 2.