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Advanced Engineering Mathematics
10th Edition
ISBN:9780470458365
Author:Erwin Kreyszig
Publisher:Erwin Kreyszig
Chapter2: Second-order Linear Odes
Section: Chapter Questions
Problem 1RQ
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Question
Please revise the solution to the problem, the board provides hints on how to solve the problem but the solution wasn't detailed enough
![Here's a transcription of the handwritten mathematical solution:
---
**Solution:**
Given, \( f \) and \( g \) are continuous at \( a \in \mathbb{R}^p \).
So, for \( \epsilon > 0 \), \( \exists \delta_1 > 0 \) such that
\[ |f(x) - f(a)| < \epsilon \quad \forall \, \|x-a\| < \delta_1 \]
For \( g \), \( \exists \delta_2 > 0 \) such that
\[ |g(x) - g(a)| < \epsilon \quad \forall \, \|x-a\| < \delta_2 \]
Thus, if we choose \( \delta = \min \{\delta_1, \delta_2\} \), then
\[ |f(x) - f(a)| < \epsilon \quad \forall \, \|x-a\| < \delta_1 \leq \delta \]
and
\[ |g(x) - g(a)| < \epsilon \quad \forall \, \|x-a\| < \delta_2 \leq \delta \]
Hence, for our chosen \( \epsilon > 0 \), we get \( \delta > 0 \)
such that
\[ |f(x) - f(a)| < \epsilon \quad \& \quad |g(x) - g(a)| < \epsilon \quad \text{whenever} \, \|x-a\| < \delta \]
**How to show:** \( f \cdot g \) is continuous at \( x = a \).
\[ \Rightarrow \text{we have} \]
\[ |(f \cdot g)(x) - (f \cdot g)(a)| = |f(x)g(x) - f(x)g(a) + f(x)g(a) - f(a)g(a)| \]
Using the triangle inequality:
\[ \leq |f(x)g(x) - f(x)g(a)| + |f(x)g(a) - f(a)g(a)| \]
\[ = |f(x)||g(x) - g(a)| + |g(a)||f(x) - f(a)| \]
**Now if** \( f \) being continuous, we know \( f \) is bounded on](/v2/_next/image?url=https%3A%2F%2Fcontent.bartleby.com%2Fqna-images%2Fquestion%2Ff5203d42-0da6-4b1b-a1a9-5b37798d2cd6%2F4dda5415-0216-4442-b486-35ac45873ea5%2F7t9lps_processed.jpeg&w=3840&q=75)
Transcribed Image Text:Here's a transcription of the handwritten mathematical solution:
---
**Solution:**
Given, \( f \) and \( g \) are continuous at \( a \in \mathbb{R}^p \).
So, for \( \epsilon > 0 \), \( \exists \delta_1 > 0 \) such that
\[ |f(x) - f(a)| < \epsilon \quad \forall \, \|x-a\| < \delta_1 \]
For \( g \), \( \exists \delta_2 > 0 \) such that
\[ |g(x) - g(a)| < \epsilon \quad \forall \, \|x-a\| < \delta_2 \]
Thus, if we choose \( \delta = \min \{\delta_1, \delta_2\} \), then
\[ |f(x) - f(a)| < \epsilon \quad \forall \, \|x-a\| < \delta_1 \leq \delta \]
and
\[ |g(x) - g(a)| < \epsilon \quad \forall \, \|x-a\| < \delta_2 \leq \delta \]
Hence, for our chosen \( \epsilon > 0 \), we get \( \delta > 0 \)
such that
\[ |f(x) - f(a)| < \epsilon \quad \& \quad |g(x) - g(a)| < \epsilon \quad \text{whenever} \, \|x-a\| < \delta \]
**How to show:** \( f \cdot g \) is continuous at \( x = a \).
\[ \Rightarrow \text{we have} \]
\[ |(f \cdot g)(x) - (f \cdot g)(a)| = |f(x)g(x) - f(x)g(a) + f(x)g(a) - f(a)g(a)| \]
Using the triangle inequality:
\[ \leq |f(x)g(x) - f(x)g(a)| + |f(x)g(a) - f(a)g(a)| \]
\[ = |f(x)||g(x) - g(a)| + |g(a)||f(x) - f(a)| \]
**Now if** \( f \) being continuous, we know \( f \) is bounded on

Transcribed Image Text:The blackboard contains mathematical proofs related to continuity of functions, specifically dealing with the continuity of the product of functions.
1. **Setup and Definitions**:
- Let \( f, g : D \subseteq \mathbb{R}^p \) be continuous functions, where \( a \in \mathbb{R}^p \).
- We establish that if both \( f \) and \( g \) are continuous at a point \( a \), then their product \( fg \) is also continuous at \( a \).
2. **Proof Structure**:
- Begin by stating that for any \( \epsilon > 0 \), there exist \( \delta_1, \delta_2 > 0 \) such that for all \( x \in \mathbb{R}^p \),
- if \( ||x - a|| < \delta_1 \), then \( ||f(x) - f(a)|| < \frac{\epsilon}{2} \).
- if \( ||x - a|| < \delta_2 \), then \( ||g(x) - g(a)|| < \frac{\epsilon}{2} \).
- Choose \( \delta = \min(\delta_1, \delta_2) \).
3. **Conclusions**:
- Illustrates the property of continuity for the product \( |f(x) \cdot g(x) - f(a) \cdot g(a)| \).
- The expression expands using the distributive property of multiplication over subtraction and absolute values.
- The conclusion follows through with bounding \( |f(x) \cdot g(x) - f(a) \cdot g(a)| \) under \( \epsilon \) when \( ||x - a|| < \delta \), proving continuity.
4. **Mathematical Expressions**:
- The board uses vectors and norms in a multi-dimensional real space \( \mathbb{R}^p \).
- The expressions include manipulatives around the concept \( ||f(x) \cdot g(x) - f(a) \cdot g(a)|| \), illustrating a breakdown into manageable components.
This proof is streamlined to validate the operational property of continuity using the structure prescribed by epsilon-delta definitions.
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