How can the recursive formula for the integral of sin^n(x)*e^-x be found?

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Homework Help Overview

The discussion revolves around finding a recursive formula for the integral \( I_{n} = \int_{0}^{\infty} \sin^{n}(x) \cdot e^{-x} \, dx \). Participants are exploring methods to derive this formula, particularly through integration by parts and substitutions.

Discussion Character

  • Exploratory, Mathematical reasoning, Problem interpretation

Approaches and Questions Raised

  • One participant attempts integration by parts and substitution but struggles to express the integral recursively. Another participant suggests performing integration by parts twice and emphasizes the choice of trigonometric terms. A third participant presents a partially recursive expression but expresses difficulty in eliminating a cosine term and questions how to prove the behavior of the integral as \( n \) approaches infinity.

Discussion Status

The discussion is ongoing, with participants sharing various approaches and insights. Some guidance has been offered regarding integration techniques, but there is no explicit consensus on how to proceed with the problem or eliminate certain terms.

Contextual Notes

Participants are working under the constraints of homework rules, which may limit the information they can use or share. There is also a mention of needing to prove a limit as \( n \) approaches infinity, adding complexity to the problem.

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Homework Statement



Find a recursive formula for

I_{n}:=\int_{0}^{\infty}\sin^{n}(x)\cdot e^{-x}\ dx

Homework Equations





The Attempt at a Solution



I wrote I_{n+1} and tried to integrate by parts and with substitution, however, I wasn't able to get a the original term so that I could write it recursively.
 
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You need to do integration by parts twice and then remember that to pick the trig terms as 'u' when integrating by parts the second time
 
I did that and got something that is partially recursive

I_{n}=n^{2}\cdot\int_{0}^{\infty}\sin^{n-2}(x)\cdot\cos^{2}(x)\cdot e^{-x}\ dx-n\cdot I_{n-2}

However, I don't know how to eliminate the cos^2 at all. Moreover, one should prove with the recursive function that when n->infinity I_n->0. But with this result I don't really know how to tackle this problem either. Somehow I'm totally stuck with this one.

thx
 
cos^2x+sin^2x=1
 

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