Beta Function of Half with Half/Proof 3

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Theorem

$\Beta \left({\dfrac 1 2, \dfrac 1 2}\right) = \pi$

where $\Beta$ denotes the Beta function.


Proof

By definition of the Beta function:

$\displaystyle \map \Beta {x, y} := \int_{\mathop \to 0}^{\mathop \to 1} t^{x - 1} \paren {1 - t}^{y - 1} \rd t$

Thus:

\(\ds \map \Beta {\dfrac 1 2, \dfrac 1 2}\) \(=\) \(\ds \int_{\mathop \to 0}^{\mathop \to 1} t^{\frac 1 2 - 1} \paren {1 - t}^{\frac 1 2 - 1} \rd t\)
\(\ds \) \(=\) \(\ds \int_{\mathop \to 0}^{\mathop \to 1} \dfrac {\rd t} {t^{1/2} \paren {1 - t}^{1/2} }\)


Let $t = u^2$.

Then:

$\rd t = 2 u \rd u$

and:

\(\ds t\) \(=\) \(\ds 0\)
\(\ds \leadsto \ \ \) \(\ds u^2\) \(=\) \(\ds 0\)
\(\ds t\) \(=\) \(\ds 1\)
\(\ds \leadsto \ \ \) \(\ds u^2\) \(=\) \(\ds 1\)
\(\ds \leadsto \ \ \) \(\ds u\) \(=\) \(\ds 1\)


and so:

\(\ds \map \Beta {\dfrac 1 2, \dfrac 1 2}\) \(=\) \(\ds \int_{\mathop \to 0}^{\mathop \to 1} \frac {2 u \rd u} {u \paren {1 - u^2}^{1/2} }\) Integration by Substitution
\(\ds \) \(=\) \(\ds 2 \int_{\mathop \to 0}^{\mathop \to 1} \frac {\rd u} {\sqrt {\paren {1 - u^2} } }\)
\(\ds \) \(=\) \(\ds 2 \bigintlimits {\arcsin u} 0 1\) Primitive of $\dfrac 1 {\sqrt {1 - u^2} }$
\(\ds \) \(=\) \(\ds 2 \times \pi / 2 - 0\)
\(\ds \) \(=\) \(\ds \pi\)

$\blacksquare$


Sources