A sort of converse to Theorem 3.5.1 (which we do not prove in this book) is this: If f: U...

A sort of converse to Theorem 3.5.1 (which we do not prove in this book) is this: If f: U ⊂ ℝn → ℝn is differentiate at a, then ∂f/∂x1, ∂f/∂x2,…, ∂f/∂xn,are all defined at a. Use this result to show that the functions in Exercise are not differentiate at the indicated point.