Primitive of Inverse Hyperbolic Cosine of x over a
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Theorem
- $\ds \int \arcosh \frac x a \rd x = x \arcosh \dfrac x a - \sqrt {x^2 - a^2} + C$
where $\arcosh$ denotes the real area hyperbolic cosine.
Corollary
- $\ds \int -\cosh^{-1} \frac x a \rd x = x \paren {-\cosh^{-1} \dfrac x a} + \sqrt {x^2 - a^2} + C$
where $-\cosh^{-1}$ denotes the negative branch of the real inverse hyperbolic cosine multifunction.
Proof
With a view to expressing the primitive in the form:
- $\ds \int u \frac {\d v} {\d x} \rd x = u v - \int v \frac {\d u} {\d x} \rd x$
let:
\(\ds u\) | \(=\) | \(\ds \arcosh \frac x a\) | ||||||||||||
\(\ds \leadsto \ \ \) | \(\ds \frac {\d u} {\d x}\) | \(=\) | \(\ds \frac 1 {\sqrt {x^2 - a^2} }\) | Derivative of $\arcosh \dfrac x a$ |
and let:
\(\ds \frac {\d v} {\d x}\) | \(=\) | \(\ds 1\) | ||||||||||||
\(\ds \leadsto \ \ \) | \(\ds v\) | \(=\) | \(\ds x\) | Primitive of Constant |
Then:
\(\ds \int \arcosh \frac x a \rd x\) | \(=\) | \(\ds x \arcosh \frac x a - \int x \paren {\frac 1 {\sqrt {x^2 - a^2} } } \rd x + C\) | Integration by Parts | |||||||||||
\(\ds \) | \(=\) | \(\ds x \arcosh \frac x a - \int \frac {x \rd x} {\sqrt {x^2 - a^2} } + C\) | simplifying | |||||||||||
\(\ds \) | \(=\) | \(\ds x \arcosh \frac x a - \sqrt {x^2 + a^2} + C\) | Primitive of $\dfrac x {\sqrt {x^2 - a^2} }$ |
$\blacksquare$
Also see
Sources
- 1968: Murray R. Spiegel: Mathematical Handbook of Formulas and Tables ... (previous) ... (next): $\S 14$: Integrals involving Inverse Hyperbolic Functions: $14.651$
- 1989: Ephraim J. Borowski and Jonathan M. Borwein: Dictionary of Mathematics ... (previous) ... (next): Appendix $2$: Table of derivatives and integrals of common functions: Inverse hyperbolic functions