Euler's Formula/Real Domain/Proof 2
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Theorem
Let $\theta \in \R$ be a real number.
Then:
- $e^{i \theta} = \cos \theta + i \sin \theta$
Proof
This:
- $e^{i \theta} = \cos \theta + i \sin \theta$
is logically equivalent to this:
- $\dfrac {\cos \theta + i \sin \theta} {e^{i \theta} } = 1$
for every $\theta$.
Note that the left expression is nowhere undefined.
Taking the derivative of this:
\(\ds \dfrac \d {\d \theta} e^{-i \theta} \paren {\cos \theta + i \sin \theta}\) | \(=\) | \(\ds e^{-i \theta} \paren {-\sin \theta + i \cos \theta} + \paren {-i e^{-i \theta} } \paren {\cos \theta + i \sin \theta}\) | Product Rule for Derivatives and Derivative of Exponential Function | |||||||||||
\(\ds \) | \(=\) | \(\ds e^{-i \theta} \paren {-\sin \theta + i \cos \theta - i \cos \theta - i^2 \sin \theta}\) | ||||||||||||
\(\ds \) | \(=\) | \(\ds e^{-i \theta} \paren {-\sin \theta + i \cos \theta - i \cos \theta + \sin \theta}\) | ||||||||||||
\(\ds \) | \(=\) | \(\ds e^{-i \theta} \paren 0\) | ||||||||||||
\(\ds \) | \(=\) | \(\ds 0\) |
Thus the expression, as a function of $\theta$, is constant and so yields the same value for every $\theta$.
We know the value at at least one point, that is, when $\theta = 0$:
- $\dfrac {\cos 0 + i \sin 0} {e^{0 i}} = \dfrac {1 + 0} 1 = 1$
Thus it is $1$ for every $\theta$, which verifies the above.
Hence the result.
$\blacksquare$