Using substitution: Describe why Sæ(5 – 2²) dx ± fu*du where u = 5 – x2.

Calculus: Early Transcendentals
8th Edition
ISBN:9781285741550
Author:James Stewart
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Chapter1: Functions And Models
Section: Chapter Questions
Problem 1RCC: (a) What is a function? What are its domain and range? (b) What is the graph of a function? (c) How...
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**Using substitution:** Describe why \(\int x (5 - x^2)^3 \, dx \neq \int u^3 \, du\) where \(u = 5 - x^2\).

### Explanation

To understand why the integrals are not equivalent, we must consider the substitution process. The given substitution is \(u = 5 - x^2\). 

1. **Differentiating**: By differentiating \(u\) with respect to \(x\), we get:
   \[
   \frac{du}{dx} = -2x \quad \Rightarrow \quad du = -2x \, dx \quad \Rightarrow \quad x \, dx = -\frac{1}{2} du
   \]

2. **Substitution in the Integral**: Substitute \(u\) and \(x \, dx\) in the original integral:
   \[
   \int x (5 - x^2)^3 \, dx = \int (5 - x^2)^3 \cdot x \, dx = \int u^3 \cdot \left(-\frac{1}{2}\right) \, du = -\frac{1}{2} \int u^3 \, du
   \]

This shows that the correct substitution results in a factor of \(-\frac{1}{2}\) in front of the integral of \(u^3\), clearly indicating that the expressions of these integrals are not directly equal without consideration of the factor introduced during substitution.
Transcribed Image Text:**Using substitution:** Describe why \(\int x (5 - x^2)^3 \, dx \neq \int u^3 \, du\) where \(u = 5 - x^2\). ### Explanation To understand why the integrals are not equivalent, we must consider the substitution process. The given substitution is \(u = 5 - x^2\). 1. **Differentiating**: By differentiating \(u\) with respect to \(x\), we get: \[ \frac{du}{dx} = -2x \quad \Rightarrow \quad du = -2x \, dx \quad \Rightarrow \quad x \, dx = -\frac{1}{2} du \] 2. **Substitution in the Integral**: Substitute \(u\) and \(x \, dx\) in the original integral: \[ \int x (5 - x^2)^3 \, dx = \int (5 - x^2)^3 \cdot x \, dx = \int u^3 \cdot \left(-\frac{1}{2}\right) \, du = -\frac{1}{2} \int u^3 \, du \] This shows that the correct substitution results in a factor of \(-\frac{1}{2}\) in front of the integral of \(u^3\), clearly indicating that the expressions of these integrals are not directly equal without consideration of the factor introduced during substitution.
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here we replace (5-x^2) with u then differentiate u, let us see

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