Use the definition of differentiability to show that is differentiable ? What does Theorem 13.4 say about the function above? Difinition of differentiability: A function f given by is differentiable at if Δ z can be written in theform. where both ε 1 and ε2–> 0 as (Δ x,Δ y) –> (0,0). Thefunction f is differentiable in a region of R if it isdifferentiable at each point in R Theorem 13.4: If f is a function of x and y, where fx andfy are continuous in an open region R, then then f isdifferentiable on R