題組內容
4. Let $f(x, y) = \begin{cases} \frac{xy}{\sqrt{x^2 + y^2}}, & (x, y) \neq (0, 0) \\ 0, & (x, y) = (0, 0) \end{cases}$
(b) \( (8\%) \) Explain \( f \) is differentiable at \( (0,0) \) or not.
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4. Let $f(x, y) = \begin{cases} \frac{xy}{\sqrt{x^2 + y^2}}, & (x, y) \neq (0, 0) \\ 0, & (x, y) = (0, 0) \end{cases}$
(b) \( (8\%) \) Explain \( f \) is differentiable at \( (0,0) \) or not.