I Find ##\int_0^π \sin^ n x dx ##

chwala
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I am looking at this definite integral; i will go slow on it with the whole intention being to understand the steps and how to apply. I will attempt to use the reduction method and i also noted that this can be approached using Beta and Gamma relationship. Its Time to upscale my intellect.
$$\int_0^π \sin^ n x dx = \int_0^π \sin^ {n-1} x\sin x dx$$

Letting ##u=\sin^{n-1} x## and ##v^{'}= \sin x## then,

$$\int_0^π \sin^ {n-1} x\sin x dx = [-\sin^ {n-1} x⋅\cos x]_0^π+ \int_0^π (n-1)\sin^{n-2} x ⋅ \cos^2 x
dx$$

$$=\int_0^π (n-1)\sin^{n-2} x ⋅ \cos^2 x dx $$

$$=\int_0^π (n-1)\sin^{n-2} x ⋅ (1- \sin^2 x) dx $$

Letting
$$I_n = \int_0^π \sin^ n x dx $$

$$I_n = (n-1)\int_0^π \left(\sin^{n-2} x - \sin^n x \right)dx $$

$$I_n = (n-1)(I_{n-2} - I_n)$$

$$nI_n = (n-1)I_{n-2}$$

Now up to this point it is quite clear. The next thing for me to check on is on conditions where ##n## is even or odd. Most importantly to understand the beta and gamma approach that forms the basis of this post.

Cheers Man!
 
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Your result with obvious
I_0=\pi, I_1=2
will give ##I_n##.
 
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Now i need to understand this $$\int_0^{0.5π} \sin^n dx = \dfrac{1}{2} β \left(\dfrac {n+1}{2}, \dfrac{1}{2}\right)$$

Let me check on the literature...
 
chwala said:
Now i need to understand this $$\int_0^{0.5π} \sin^n dx = \dfrac{1}{2} β \left(\dfrac {n+1}{2}, \dfrac{1}{2}\right)$$

Let me check on the literature...

Substitute u = \sin^2 x, and use the definition <br /> \beta(p,q) = \int_0^1 u^{p-1}(1-u)^{q-1}\,du.

This is a special case of <br /> \int_0^{\pi/2} \cos^n x \sin^m x\,dx = \frac12 \beta\left( \frac{n+1}{2}, \frac{m + 1}{2}\right)
 
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## \begin{align}
I_n&=\int_{0}^{\frac\pi2}sin^nx dx\nonumber\\
&=\frac12\int_{0}^{\frac\pi2}\frac {sin^nx}{sinx}\frac1{cosx}2\sin x\cos xdx\nonumber\\
&=\frac12\int_{0}^{\frac\pi2}(sin^2x)^\frac{n-1}2(1-sin^2x)^{-\frac12}2\sin x\cos xdx\nonumber\\
\end{align} ##

## \begin{align}
&u=sin^2x\nonumber\\
&du=2\sin x\cos x dx\nonumber\\
\end{align} ##

## \begin{align}
I_n&=\frac12\int_{0}^{1}u^\frac{n-1}2(1-u)^{-\frac12}du\nonumber\\
&=\frac12\beta(\frac{n+1}2,\frac12)\nonumber\\
\end{align} ##

This is actually an integration by substitution method which gives a beta function as a result.

The final result is:
$$ \int_{0}^{\pi}sin^nx dx=\beta(\frac{n+1}2,\frac12) $$
 
pasmith said:
Substitute u = \sin^2 x, and use the definition <br /> \beta(p,q) = \int_0^1 u^{p-1}(1-u)^{q-1}\,du.

This is a special case of <br /> \int_0^{\pi/2} \cos^n x \sin^m x\,dx = \frac12 \beta\left( \frac{n+1}{2}, \frac{m + 1}{2}\right)
aaaaah @pasmith now i see...

from,

##\int \sin^{2m-1} θ \cos ^{2n-1} θ dθ= \dfrac{1}{2} β (m,n)## This is the key.

Noting that the Beta function and gamma function are related by,

##β(m,n) = \dfrac {Γ(m) Γ(n)}{Γ(m+n)}##

then in our case,by considering the integral we have,

##\cos^{2n-1} θ= 1## implying that

##n=\dfrac{1}{2}##

and letting, ##\sin^{2m-1} θ = \sin^n θ##
##2m-1=n##
##m=\dfrac{n+1}{2}##

from that point of view the problem can be handled...
 
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