Laplace Transform of Sine/Proof 2
Jump to navigation
Jump to search
Theorem
Let $\sin$ denote the real sine function.
Let $\laptrans f$ denote the Laplace transform of a real function $f$.
Then:
- $\laptrans {\sin at} = \dfrac a {s^2 + a^2}$
where $a \in \R_{>0}$ is constant, and $\map \Re s > 0$.
Proof
\(\ds \laptrans {e^{i a t} }\) | \(=\) | \(\ds \frac 1 {s - i a}\) | Laplace Transform of Exponential | |||||||||||
\(\ds \) | \(=\) | \(\ds \frac {s + i a} {s^2 + a^2}\) | multiplying top and bottom by $s + i a$ |
Also:
\(\ds \laptrans {e^{i a t} }\) | \(=\) | \(\ds \laptrans {\cos a t + i \sin a t}\) | Euler's Formula | |||||||||||
\(\ds \) | \(=\) | \(\ds \laptrans {\cos a t} + i \laptrans {\sin a t}\) | Linear Combination of Laplace Transforms |
So:
\(\ds \laptrans {\sin a t}\) | \(=\) | \(\ds \map \Im {\laptrans {e^{i a t} } }\) | ||||||||||||
\(\ds \) | \(=\) | \(\ds \map \Im {\frac {s + i a} {s^2 + a^2} }\) | ||||||||||||
\(\ds \) | \(=\) | \(\ds \frac a {s^2 + a^2}\) |
$\blacksquare$
Sources
- 1965: Murray R. Spiegel: Theory and Problems of Laplace Transforms ... (previous) ... (next): Chapter $1$: The Laplace Transform: Solved Problems: Laplace Transforms of some Elementary Functions: $2$