Prove the following limit using the limit c im,(x³ – x²) =+∞

Advanced Engineering Mathematics
10th Edition
ISBN:9780470458365
Author:Erwin Kreyszig
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Chapter2: Second-order Linear Odes
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Class: Mathematical Analysis/Real Analysis

**Proof of Limit Using the Limit Definition**

**Problem Statement:**

Prove the following limit using the limit definition:

\[
\lim_{x \to \infty} (x^3 - x^2) = +\infty
\]

**Analysis:**

To prove this limit using the definition, we must show that for every positive number \( M \), there exists a positive number \( N \) such that for all \( x > N \), \( x^3 - x^2 > M \).

**Proof Outline:**

1. **Simplify the Expression:**
   - Consider the function \( f(x) = x^3 - x^2 \).
   - Factor the expression: \( f(x) = x^2(x - 1) \).

2. **Determine Bounds:**
   - As \( x \to \infty \), both \( x^2 \) and \( x \) grow without bound.
   - Therefore, \( x^3 - x^2 = x^2(x - 1) \) grows without bound as \( x \to \infty \).

3. **Find Suitable N:**
   - For \( x > 1 \), \( x - 1 > 0 \).
   - Choose \( N \) such that \( x^2(x - 1) > M \).
   - Simplification suggests \( x^3 - x^2 = x^2 (x - 1) > M \).

4. **Conclusion:**
   - For any given \( M \), select \( N \) such that \( N > \max(1, \sqrt{M}) \).
   - Thus, for all \( x > N \), \( x^2(x - 1) > M \).

This concludes that:

\[
\lim_{x \to \infty} (x^3 - x^2) = +\infty
\]

This proof effectively demonstrates the behavior of the function as \( x \) approaches infinity and confirms the limit using the formal definition.
Transcribed Image Text:**Proof of Limit Using the Limit Definition** **Problem Statement:** Prove the following limit using the limit definition: \[ \lim_{x \to \infty} (x^3 - x^2) = +\infty \] **Analysis:** To prove this limit using the definition, we must show that for every positive number \( M \), there exists a positive number \( N \) such that for all \( x > N \), \( x^3 - x^2 > M \). **Proof Outline:** 1. **Simplify the Expression:** - Consider the function \( f(x) = x^3 - x^2 \). - Factor the expression: \( f(x) = x^2(x - 1) \). 2. **Determine Bounds:** - As \( x \to \infty \), both \( x^2 \) and \( x \) grow without bound. - Therefore, \( x^3 - x^2 = x^2(x - 1) \) grows without bound as \( x \to \infty \). 3. **Find Suitable N:** - For \( x > 1 \), \( x - 1 > 0 \). - Choose \( N \) such that \( x^2(x - 1) > M \). - Simplification suggests \( x^3 - x^2 = x^2 (x - 1) > M \). 4. **Conclusion:** - For any given \( M \), select \( N \) such that \( N > \max(1, \sqrt{M}) \). - Thus, for all \( x > N \), \( x^2(x - 1) > M \). This concludes that: \[ \lim_{x \to \infty} (x^3 - x^2) = +\infty \] This proof effectively demonstrates the behavior of the function as \( x \) approaches infinity and confirms the limit using the formal definition.
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