5. Find L {e" cos?t cos t

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ISBN:9780470458365
Author:Erwin Kreyszig
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**Problem 5**: Find \(\mathcal{L}\left\{ e^{7t} \cos^2 t \right\}\)

This problem requires finding the Laplace transform of the given function \( e^{7t} \cos^2 t \). The Laplace transform is an integral transform used to convert a function of a real variable \( t \) (often time) to a function of a complex variable \( s \) (complex frequency).

### Steps to Solve the Problem:

1. **Express the Trigonometric Function with Double Angle Formulas**:
   Use the identity for the double angle of cosine:
   \[
   \cos^2 t = \frac{1 + \cos(2t)}{2}
   \]
   which simplifies the given function:
   \[
   e^{7t} \cos^2 t = e^{7t} \cdot \frac{1 + \cos(2t)}{2}
   \]

2. **Rewrite the Function**:
   Rewrite the function to a more manageable form:
   \[
   e^{7t} \cos^2 t = \frac{1}{2} e^{7t} + \frac{1}{2} e^{7t} \cos(2t)
   \]

3. **Apply Linearity of the Laplace Transform**:
   The Laplace transform \(\mathcal{L}\) is linear, so apply it to each term separately:
   \[
   \mathcal{L}\left\{ \frac{1}{2} e^{7t} + \frac{1}{2} e^{7t} \cos(2t) \right\} = \frac{1}{2} \mathcal{L}\left\{ e^{7t} \right\} + \frac{1}{2} \mathcal{L}\left\{ e^{7t} \cos(2t) \right\}
   \]

4. **Find the Laplace Transforms**:

   - For \(\mathcal{L}\{ e^{7t} \}\):
     \[
     \mathcal{L}\left\{ e^{7t} \right\} = \frac{1}{s - 7}, \quad \text{for } s > 7
     \]

   - For \
Transcribed Image Text:**Problem 5**: Find \(\mathcal{L}\left\{ e^{7t} \cos^2 t \right\}\) This problem requires finding the Laplace transform of the given function \( e^{7t} \cos^2 t \). The Laplace transform is an integral transform used to convert a function of a real variable \( t \) (often time) to a function of a complex variable \( s \) (complex frequency). ### Steps to Solve the Problem: 1. **Express the Trigonometric Function with Double Angle Formulas**: Use the identity for the double angle of cosine: \[ \cos^2 t = \frac{1 + \cos(2t)}{2} \] which simplifies the given function: \[ e^{7t} \cos^2 t = e^{7t} \cdot \frac{1 + \cos(2t)}{2} \] 2. **Rewrite the Function**: Rewrite the function to a more manageable form: \[ e^{7t} \cos^2 t = \frac{1}{2} e^{7t} + \frac{1}{2} e^{7t} \cos(2t) \] 3. **Apply Linearity of the Laplace Transform**: The Laplace transform \(\mathcal{L}\) is linear, so apply it to each term separately: \[ \mathcal{L}\left\{ \frac{1}{2} e^{7t} + \frac{1}{2} e^{7t} \cos(2t) \right\} = \frac{1}{2} \mathcal{L}\left\{ e^{7t} \right\} + \frac{1}{2} \mathcal{L}\left\{ e^{7t} \cos(2t) \right\} \] 4. **Find the Laplace Transforms**: - For \(\mathcal{L}\{ e^{7t} \}\): \[ \mathcal{L}\left\{ e^{7t} \right\} = \frac{1}{s - 7}, \quad \text{for } s > 7 \] - For \
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