Cosine in terms of Hyperbolic Cosine
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Theorem
Let $z \in \C$ be a complex number.
Then:
- $\cos z = \map \cosh {i z}$
where:
- $\cos$ denotes the complex cosine
- $\cosh$ denotes the hyperbolic cosine
- $i$ is the imaginary unit: $i^2 = -1$.
Proof
\(\ds \map \cosh {i z}\) | \(=\) | \(\ds \frac {e^{i z} + e^{-i z} } 2\) | Definition of Hyperbolic Cosine | |||||||||||
\(\ds \) | \(=\) | \(\ds \cos z\) | Euler's Cosine Identity |
$\blacksquare$
Also see
- Sine in terms of Hyperbolic Sine
- Tangent in terms of Hyperbolic Tangent
- Cotangent in terms of Hyperbolic Cotangent
- Secant in terms of Hyperbolic Secant
- Cosecant in terms of Hyperbolic Cosecant
Sources
- 1960: Walter Ledermann: Complex Numbers ... (previous) ... (next): $\S 4.5$. The Functions $e^z$, $\cos z$, $\sin z$: $(4.22)$
- 1968: Murray R. Spiegel: Mathematical Handbook of Formulas and Tables ... (previous) ... (next): $\S 8$: Hyperbolic Functions: $8.81$: Relationship between Hyperbolic and Trigonometric Functions
- 1969: J.C. Anderson, D.M. Hum, B.G. Neal and J.H. Whitelaw: Data and Formulae for Engineering Students (2nd ed.) ... (previous) ... (next): $4.$ Mathematics: $4.3$ Trigonometric identities and hyperbolic functions
- 1981: Murray R. Spiegel: Theory and Problems of Complex Variables (SI ed.) ... (previous) ... (next): $2$: Functions, Limits and Continuity: The Elementary Functions: $5$
- 1998: David Nelson: The Penguin Dictionary of Mathematics (2nd ed.) ... (previous) ... (next): hyperbolic functions
- 2008: David Nelson: The Penguin Dictionary of Mathematics (4th ed.) ... (previous) ... (next): hyperbolic functions