xy + Let f: R2 → R where f(x, y) = 0 if (x, y) = (0,0) and f(x, y) = 72²² if (x, y) = (0,0). (a) Prove that Duf (0,0) = 0 if u = (1,0). (b) Prove that Duf(0, 0) = 0 if u = (0, 1). (c) Prove that Duf(0, 0) does not exist if u = (a, b) where ab 0. (d) Prove that Df(0, 0) does not exist (this can be done by proving that f is not continuous at (0, 0)).

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Chapter2: Second-order Linear Odes
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Real Analysis II Please solve 1b,c&d
**Title: Analysis of a Multivariable Function**

---

**Problem Statement:**

Let \( f : \mathbb{R}^2 \to \mathbb{R} \) where 

\[ f(x, y) = 0 \quad \text{if } (x, y) = (0, 0) \]

and 

\[ f(x, y) = \frac{xy}{x^2 + y^2} \quad \text{if } (x, y) \neq (0, 0). \]

1. Prove the following:

   (a) The directional derivative \( D_{\mathbf{u}} f(0, 0) = 0 \) if \( \mathbf{u} = (1, 0) \).

   (b) The directional derivative \( D_{\mathbf{u}} f(0, 0) = 0 \) if \( \mathbf{u} = (0, 1) \).

   (c) The directional derivative \( D_{\mathbf{u}} f(0, 0) \) does not exist if \( \mathbf{u} = (a, b) \) where \( ab \neq 0 \).

   (d) The derivative \( Df(0, 0) \) does not exist, shown by proving \( f \) is not continuous at \( (0, 0) \).

---

**Solution Outline:**

- **Part (a):** For \(\mathbf{u} = (1,0)\), calculate the limit:

  \[
  \lim_{h \to 0} \frac{f(h, 0) - f(0, 0)}{h} = \lim_{h \to 0} \frac{0}{h} = 0.
  \]

- **Part (b):** For \(\mathbf{u} = (0,1)\), calculate the limit:

  \[
  \lim_{k \to 0} \frac{f(0, k) - f(0, 0)}{k} = \lim_{k \to 0} \frac{0}{k} = 0.
  \]

- **Part (c):** For \(\mathbf{u} = (a, b)\) with \(ab \neq 0\), evaluate:
Transcribed Image Text:**Title: Analysis of a Multivariable Function** --- **Problem Statement:** Let \( f : \mathbb{R}^2 \to \mathbb{R} \) where \[ f(x, y) = 0 \quad \text{if } (x, y) = (0, 0) \] and \[ f(x, y) = \frac{xy}{x^2 + y^2} \quad \text{if } (x, y) \neq (0, 0). \] 1. Prove the following: (a) The directional derivative \( D_{\mathbf{u}} f(0, 0) = 0 \) if \( \mathbf{u} = (1, 0) \). (b) The directional derivative \( D_{\mathbf{u}} f(0, 0) = 0 \) if \( \mathbf{u} = (0, 1) \). (c) The directional derivative \( D_{\mathbf{u}} f(0, 0) \) does not exist if \( \mathbf{u} = (a, b) \) where \( ab \neq 0 \). (d) The derivative \( Df(0, 0) \) does not exist, shown by proving \( f \) is not continuous at \( (0, 0) \). --- **Solution Outline:** - **Part (a):** For \(\mathbf{u} = (1,0)\), calculate the limit: \[ \lim_{h \to 0} \frac{f(h, 0) - f(0, 0)}{h} = \lim_{h \to 0} \frac{0}{h} = 0. \] - **Part (b):** For \(\mathbf{u} = (0,1)\), calculate the limit: \[ \lim_{k \to 0} \frac{f(0, k) - f(0, 0)}{k} = \lim_{k \to 0} \frac{0}{k} = 0. \] - **Part (c):** For \(\mathbf{u} = (a, b)\) with \(ab \neq 0\), evaluate:
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