# Definite Integral from 0 to Half Pi of Even Power of Sine x/Proof 1

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## Theorem

- $\displaystyle \int_0^{\frac \pi 2} \sin^{2 n} x \rd x = \dfrac {\paren {2 n}!} {\paren {2^n n!}^2} \dfrac \pi 2$

## Proof

Let $I_n = \displaystyle \int_0^{\frac \pi 2} \sin^n x \rd x$.

Then:

\(\displaystyle I_{2 n}\) | \(=\) | \(\displaystyle \frac {2 n - 1} {2 n} I_{2 n - 2}\) | Reduction Formula for Definite Integral of Power of Sine | ||||||||||

\(\displaystyle \) | \(=\) | \(\displaystyle \frac {\paren {2 n - 1} \paren {2 n - 3} } {2 n \paren {2 n - 2} } I_{2 n - 4}\) | Reduction Formula for Definite Integral of Power of Sine again | ||||||||||

\(\displaystyle \) | \(=\) | \(\displaystyle \frac {\paren {2 n - 1} \paren {2 n - 3} \cdots 1} {2 n \paren {2 n - 2} \cdots 2} I_0\) | Reduction Formula for Definite Integral of Power of Sine until the end | ||||||||||

\(\displaystyle \) | \(=\) | \(\displaystyle \frac {\paren {2 n - 1} \paren {2 n - 3} \cdots 1} {2 n \paren {2 n - 2} \cdots 2} \int_0^{\pi / 2} \rd x\) | Definition of $I_n$ | ||||||||||

\(\displaystyle \) | \(=\) | \(\displaystyle \frac {\paren {2 n - 1} \paren {2 n - 3} \cdots 1} {2 n \paren {2 n - 2} \cdots 2} \bigintlimits x 0 {\pi / 2}\) | Integral of Constant | ||||||||||

\(\displaystyle \) | \(=\) | \(\displaystyle \frac {\paren {2 n - 1} \paren {2 n - 3} \cdots 1} {2 n \paren {2 n - 2} \cdots 2} \frac \pi 2\) | Integral of Constant | ||||||||||

\(\displaystyle \) | \(=\) | \(\displaystyle \frac {2n \paren {2 n - 1} \paren {2 n - 2} \paren {2 n - 3} \cdots 2 \cdot 1} {\paren {2 n}^2 \paren {2 n - 2}^2 \cdots 2^2} \frac \pi 2\) | multiplying top and bottom by bottom | ||||||||||

\(\displaystyle \) | \(=\) | \(\displaystyle \frac {2n \paren {2 n - 1} \paren {2 n - 2} \paren {2 n - 3} \cdots 2 \cdot 1} {\paren {2^n}^2 n^2 \paren {n - 1}^2 \cdots 1^2} \frac \pi 2\) | extracting factor of $\paren {2^n}^2$ from the bottom | ||||||||||

\(\displaystyle \) | \(=\) | \(\displaystyle \frac {\paren {2 n}!} {\paren {2^n}^2 \paren {n!}^2} \frac \pi 2\) | Definition of Factorial | ||||||||||

\(\displaystyle \) | \(=\) | \(\displaystyle \frac {\paren {2 n}!} {\paren {2^n n!}^2} \frac \pi 2\) | rearranging |

$\blacksquare$