Euler's Formula/Real Domain/Proof 4
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Theorem
Let $\theta \in \R$ be a real number.
Then:
- $e^{i \theta} = \cos \theta + i \sin \theta$
Proof
Note that the following proof, as written, only holds for real $\theta$.
Define:
- $\map x \theta = e^{i \theta}$
- $\map y \theta = \cos \theta + i \sin \theta$
Consider first $\theta \ge 0$.
Taking Laplace transforms:
\(\ds \map {\laptrans {\map x \theta} } s\) | \(=\) | \(\ds \map {\laptrans {e^{i \theta} } } s\) | ||||||||||||
\(\ds \) | \(=\) | \(\ds \frac 1 {s - i}\) | Laplace Transform of Exponential | |||||||||||
\(\ds \map {\laptrans {\map y \theta} } s\) | \(=\) | \(\ds \map {\laptrans {\cos \theta + i \sin \theta} } s\) | ||||||||||||
\(\ds \) | \(=\) | \(\ds \map {\laptrans {\cos \theta} } s + i \, \map {\laptrans {\sin \theta} } s\) | Linear Combination of Laplace Transforms | |||||||||||
\(\ds \) | \(=\) | \(\ds \frac s {s^2 + 1} + \frac i {s^2 + 1}\) | Laplace Transform of Cosine, Laplace Transform of Sine | |||||||||||
\(\ds \) | \(=\) | \(\ds \frac {s + i} {\paren {s + i} \paren {s - i} }\) | ||||||||||||
\(\ds \) | \(=\) | \(\ds \frac 1 {s - i}\) |
So $x$ and $y$ have the same Laplace transform for $\theta \ge 0$.
Now define $\tau = -\theta, \sigma = -s$, and consider $\theta < 0$ so that $\tau > 0$.
Taking Laplace transforms of $\map x \tau$ and $\map y \tau$:
\(\ds \map {\laptrans {\map x \tau} } \sigma\) | \(=\) | \(\ds \frac 1 {\sigma - i}\) | from above | |||||||||||
\(\ds \map {\laptrans {\map y \tau} } \sigma\) | \(=\) | \(\ds \frac 1 {\sigma - i}\) | from above |
So $\map x \theta$ and $\map y \theta$ have the same Laplace transforms for $\theta < 0$.
The result follows from Injectivity of Laplace Transform.
$\blacksquare$