Find the Laplace transform L{sin2tcost2t}.

Advanced Engineering Mathematics
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ISBN:9780470458365
Author:Erwin Kreyszig
Publisher:Erwin Kreyszig
Chapter2: Second-order Linear Odes
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**Problem Statement:**

Find the Laplace transform \( \mathcal{L} \{ \sin 2t \cos 2t \} \).

**Solution:**

To find the Laplace transform of the given function \( \sin 2t \cos 2t \), we can use trigonometric identities to simplify the expression. Specifically, we can use the product-to-sum identities, which state:

\[ \sin A \cos B = \frac{1}{2} [\sin (A+B) + \sin (A-B)] \]

For the given problem, \( A = 2t \) and \( B = 2t \), which gives:

\[ \sin 2t \cos 2t = \frac{1}{2} [\sin (2t + 2t) + \sin (2t - 2t)] \]
\[ = \frac{1}{2} [\sin 4t + \sin 0] \]
\[ = \frac{1}{2} \sin 4t \]

Now we need to find \( \mathcal{L} \left\{ \frac{1}{2} \sin 4t \right\} \).

The Laplace transform of \( \sin kt \) is given by:

\[ \mathcal{L} \{\sin kt\} = \frac{k}{s^2 + k^2} \]

In this case, \( k = 4 \), so:

\[ \mathcal{L} \{ \sin 4t \} = \frac{4}{s^2 + 16} \]

Since:

\[ \mathcal{L} \left\{ \frac{1}{2} \sin 4t \right\} = \frac{1}{2} \mathcal{L} \{ \sin 4t \} \]

Therefore:

\[ \mathcal{L} \left\{ \frac{1}{2} \sin 4t \right\} = \frac{1}{2} \cdot \frac{4}{s^2 + 16} = \frac{2}{s^2 + 16} \]

Thus:

\[ \mathcal{L} \{ \sin 2t \cos 2t \} = \frac{2}{s^2 +
Transcribed Image Text:**Problem Statement:** Find the Laplace transform \( \mathcal{L} \{ \sin 2t \cos 2t \} \). **Solution:** To find the Laplace transform of the given function \( \sin 2t \cos 2t \), we can use trigonometric identities to simplify the expression. Specifically, we can use the product-to-sum identities, which state: \[ \sin A \cos B = \frac{1}{2} [\sin (A+B) + \sin (A-B)] \] For the given problem, \( A = 2t \) and \( B = 2t \), which gives: \[ \sin 2t \cos 2t = \frac{1}{2} [\sin (2t + 2t) + \sin (2t - 2t)] \] \[ = \frac{1}{2} [\sin 4t + \sin 0] \] \[ = \frac{1}{2} \sin 4t \] Now we need to find \( \mathcal{L} \left\{ \frac{1}{2} \sin 4t \right\} \). The Laplace transform of \( \sin kt \) is given by: \[ \mathcal{L} \{\sin kt\} = \frac{k}{s^2 + k^2} \] In this case, \( k = 4 \), so: \[ \mathcal{L} \{ \sin 4t \} = \frac{4}{s^2 + 16} \] Since: \[ \mathcal{L} \left\{ \frac{1}{2} \sin 4t \right\} = \frac{1}{2} \mathcal{L} \{ \sin 4t \} \] Therefore: \[ \mathcal{L} \left\{ \frac{1}{2} \sin 4t \right\} = \frac{1}{2} \cdot \frac{4}{s^2 + 16} = \frac{2}{s^2 + 16} \] Thus: \[ \mathcal{L} \{ \sin 2t \cos 2t \} = \frac{2}{s^2 +
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