P 3. Evaluate the integral fx cos(3x²) dx as an infinite series. (You do not need to find the radius of convergence.)

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### Problem 3

**Objective**: Evaluate the integral \(\int x^4 \cos (3x^2) \, dx\) as an infinite series. (You do not need to find the radius of convergence.)

---

**Solution Approach**:

1. **Express the Integrand Using a Series**: Begin by expanding \(\cos(3x^2)\) into its Taylor series. The general Taylor series expansion for \(\cos(u)\) is given by:
   \[
   \cos(u) = \sum_{n=0}^{\infty} \frac{(-1)^n u^{2n}}{(2n)!}
   \]
   Here, \(u = 3x^2\).

2. **Substitute into the Integral**: Replace \(\cos(3x^2)\) in the integral with its series expansion:
   \[
   \cos(3x^2) = \sum_{n=0}^{\infty} \frac{(-1)^n (3x^2)^{2n}}{(2n)!}
   \]
   Therefore, our integral becomes:
   \[
   \int x^4 \cos (3x^2) \, dx = \int x^4 \left( \sum_{n=0}^{\infty} \frac{(-1)^n (3x^2)^{2n}}{(2n)!} \right) \, dx
   \]

3. **Combine and Simplify**: Simplify the expression inside the integral:
   \[
   \int x^4 \sum_{n=0}^{\infty} \frac{(-1)^n (3^{2n} x^{4n})}{(2n)!} \, dx = \sum_{n=0}^{\infty} \int x^4 \frac{(-1)^n 3^{2n} x^{4n}}{(2n)!} \, dx
   \]
   This equation follows by interchanging the summation and integration (justified if the series converges uniformly).

4. **Simplify Further**: Combine \(x^4\) and \(x^{4n}\):
   \[
   \sum_{n=0}^{\infty} \frac{(-1)^n 3^{2n}}{(
Transcribed Image Text:### Problem 3 **Objective**: Evaluate the integral \(\int x^4 \cos (3x^2) \, dx\) as an infinite series. (You do not need to find the radius of convergence.) --- **Solution Approach**: 1. **Express the Integrand Using a Series**: Begin by expanding \(\cos(3x^2)\) into its Taylor series. The general Taylor series expansion for \(\cos(u)\) is given by: \[ \cos(u) = \sum_{n=0}^{\infty} \frac{(-1)^n u^{2n}}{(2n)!} \] Here, \(u = 3x^2\). 2. **Substitute into the Integral**: Replace \(\cos(3x^2)\) in the integral with its series expansion: \[ \cos(3x^2) = \sum_{n=0}^{\infty} \frac{(-1)^n (3x^2)^{2n}}{(2n)!} \] Therefore, our integral becomes: \[ \int x^4 \cos (3x^2) \, dx = \int x^4 \left( \sum_{n=0}^{\infty} \frac{(-1)^n (3x^2)^{2n}}{(2n)!} \right) \, dx \] 3. **Combine and Simplify**: Simplify the expression inside the integral: \[ \int x^4 \sum_{n=0}^{\infty} \frac{(-1)^n (3^{2n} x^{4n})}{(2n)!} \, dx = \sum_{n=0}^{\infty} \int x^4 \frac{(-1)^n 3^{2n} x^{4n}}{(2n)!} \, dx \] This equation follows by interchanging the summation and integration (justified if the series converges uniformly). 4. **Simplify Further**: Combine \(x^4\) and \(x^{4n}\): \[ \sum_{n=0}^{\infty} \frac{(-1)^n 3^{2n}}{(
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