47. Show that y = x³ + 3x + 1 satisfies y"" + xy" - 2y = 0.

Calculus: Early Transcendentals
8th Edition
ISBN:9781285741550
Author:James Stewart
Publisher:James Stewart
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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### Problem 47

Show that \( y = x^3 + 3x + 1 \) satisfies \( y''' + xy'' - 2y' = 0 \).

---

To solve this, we need to take the necessary derivatives of \( y \) and then substitute them into the given differential equation to verify the solution.

1. **First Derivative \( y' \)**:
   \[
   y = x^3 + 3x + 1
   \]
   \[
   y' = \frac{d}{dx}(x^3 + 3x + 1) = 3x^2 + 3
   \]

2. **Second Derivative \( y'' \)**:
   \[
   y'' = \frac{d}{dx}(3x^2 + 3) = 6x
   \]

3. **Third Derivative \( y''' \)**:
   \[
   y''' = \frac{d}{dx}(6x) = 6
   \]

Now we substitute \( y''' \), \( y'' \), and \( y' \) into the differential equation \( y''' + xy'' - 2y' \):

\[
y''' + xy'' - 2y' = 6 + x(6x) - 2(3x^2 + 3)
\]

Simplify the expression:

\[
6 + 6x^2 - 6x^2 - 6
\]

Combine like terms:

\[
6 - 6 + 6x^2 - 6x^2 = 0
\]

Therefore, \( y''' + xy'' - 2y' = 0 \) is verified, confirming that \( y = x^3 + 3x + 1 \) is a solution to the differential equation.
Transcribed Image Text:### Problem 47 Show that \( y = x^3 + 3x + 1 \) satisfies \( y''' + xy'' - 2y' = 0 \). --- To solve this, we need to take the necessary derivatives of \( y \) and then substitute them into the given differential equation to verify the solution. 1. **First Derivative \( y' \)**: \[ y = x^3 + 3x + 1 \] \[ y' = \frac{d}{dx}(x^3 + 3x + 1) = 3x^2 + 3 \] 2. **Second Derivative \( y'' \)**: \[ y'' = \frac{d}{dx}(3x^2 + 3) = 6x \] 3. **Third Derivative \( y''' \)**: \[ y''' = \frac{d}{dx}(6x) = 6 \] Now we substitute \( y''' \), \( y'' \), and \( y' \) into the differential equation \( y''' + xy'' - 2y' \): \[ y''' + xy'' - 2y' = 6 + x(6x) - 2(3x^2 + 3) \] Simplify the expression: \[ 6 + 6x^2 - 6x^2 - 6 \] Combine like terms: \[ 6 - 6 + 6x^2 - 6x^2 = 0 \] Therefore, \( y''' + xy'' - 2y' = 0 \) is verified, confirming that \( y = x^3 + 3x + 1 \) is a solution to the differential equation.
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