Definite Integral from 0 to 1 of Arcsine of x over x
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Theorem
- $\ds \int_0^1 \frac {\arcsin x} x = \frac \pi 2 \ln 2$
Proof
Let:
- $x = \sin \theta$
By Derivative of Sine Function, we have:
- $\dfrac {\d x} {\d \theta} = \cos \theta$
We have, by Arcsine of Zero is Zero:
- as $x \to 0$, $\theta \to \arcsin 0 = 0$.
By Arcsine of One is Half Pi, we have:
- as $x \to 1$, $\theta \to \arcsin 1 = \dfrac \pi 2$.
We have:
\(\ds \int_0^1 \frac {\arcsin x} x \rd x\) | \(=\) | \(\ds \int_0^{\pi/2} \frac {\cos \theta \map \arcsin {\sin \theta} } {\sin \theta} \rd \theta\) | substituting $x = \sin \theta$ | |||||||||||
\(\ds \) | \(=\) | \(\ds \int_0^{\pi/2} \theta \cot \theta \rd \theta\) | Definition of Real Arcsine, Definition of Real Cotangent Function |
By Primitive of Cotangent Function:
- $\ds \int \cot \theta \rd \theta = \map \ln {\sin \theta} + C$
So:
\(\ds \int_0^{\pi/2} \theta \cot \theta \rd \theta\) | \(=\) | \(\ds \bigintlimits {\theta \map \ln {\sin \theta} } 0 {\frac \pi 2} - \int_0^{\pi/2} \map \ln {\sin \theta} \rd \theta\) | Integration by Parts | |||||||||||
\(\ds \) | \(=\) | \(\ds \frac \pi 2 \map \ln {\sin \frac \pi 2} - \lim_{\theta \to 0^+} \paren {\theta \map \ln {\sin \theta} } + \frac \pi 2 \ln 2\) | Definite Integral from 0 to $\dfrac \pi 2$ of $\map \ln {\sin x}$ | |||||||||||
\(\ds \) | \(=\) | \(\ds -\lim_{\theta \to 0^+} \paren {\theta \map \ln {\sin \theta} } + \frac \pi 2 \ln 2\) | Sine of Right Angle, Natural Logarithm of 1 is 0 |
We have:
\(\ds \lim_{\theta \to 0^+} \paren {\theta \map \ln {\sin \theta} }\) | \(=\) | \(\ds \lim_{\theta \to 0^+} \paren {\theta \map \ln {\frac {\sin \theta} \theta \theta} }\) | ||||||||||||
\(\ds \) | \(=\) | \(\ds \lim_{\theta \to 0^+} \paren {\theta \map \ln {\frac {\sin \theta} \theta} } + \lim_{\theta \to 0^+} \theta \ln \theta\) | Sum of Logarithms, Sum Rule for Limits of Real Functions | |||||||||||
\(\ds \) | \(=\) | \(\ds \paren {\lim_{\theta \to 0^+} \theta} \paren {\map \ln {\lim_{\theta \to 0^+} \frac {\sin \theta} \theta} } + \lim_{\theta \to 0^+} \theta \ln \theta\) | Product Rule for Limits of Real Functions | |||||||||||
\(\ds \) | \(=\) | \(\ds 0 \ln 1 + 0\) | Limit of $\dfrac {\sin x} x$ at Zero, Limit of $x^n \paren {\ln x}^m$ | |||||||||||
\(\ds \) | \(=\) | \(\ds 0\) |
giving:
- $\ds \int_0^{\pi/2} \theta \cot \theta \rd \theta = \frac \pi 2 \ln 2$
hence the result.
$\blacksquare$
Sources
- 1968: Murray R. Spiegel: Mathematical Handbook of Formulas and Tables ... (previous) ... (next): $\S 15$: Definite Integrals involving Trigonometric Functions: $15.64$