I am given a function: $$ f(x,y) = \begin{cases} 8 & (x,y)=(2,0) \\ 4x+y+\frac{xy-2y}{\sqrt{x^2+4x+y^2+4}} & (x,y)\neq(2,0) \end{cases} $$
I have to determine if this function is differentiable. I am not sure how to approach this problem. Thanks for any help.