Solution for Tricky Definite Integral: How to Find I in Terms of A"

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Discussion Overview

The discussion revolves around finding a solution for the definite integral I = 0∫1 xf(x)dx in terms of a known quantity A, where A = 0∫1 f(x)dx. Participants explore various methods and constraints related to the function f(x), which is described as a transcendental function with specific boundary conditions.

Discussion Character

  • Exploratory, Technical explanation, Debate/contested, Mathematical reasoning

Main Points Raised

  • One participant seeks guidance on solving the integral I in terms of A, given the constraints on f(x).
  • Another participant argues that without more details about f(x), finding a solution is impossible, as different functions can yield the same A while resulting in different I.
  • A participant provides a specific form of f(x) as a transcendental function but expresses uncertainty about its usefulness in solving the integral.
  • Some participants suggest that if f(x) has a power series expansion, it might be possible to approximate I, though doubts about the existence of an analytic solution are raised.
  • Multiple participants mention that Wolfram Alpha provides a complex form of f(x) and relates it to Lambert's W function, indicating the potential difficulty in finding a simpler solution.
  • One participant proposes using rotation of coordinates as a potential method to solve the integral.
  • Another participant suggests an integration by parts approach but expresses uncertainty about its effectiveness.
  • A later reply attempts to provide a method involving setting u=x and dv=f(x)dx, but the participant indicates they cannot solve it with their current knowledge.

Areas of Agreement / Disagreement

Participants generally agree that the lack of specific details about f(x) complicates finding a solution. There are multiple competing views on potential methods to approach the problem, but no consensus on a definitive solution exists.

Contextual Notes

The discussion highlights limitations related to the unknown nature of f(x) and the complexity of its form, which may affect the ability to derive a solution for I.

jam_27
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Am trying to get a solution to the definite integral below. Looking for some direction.

I = 01 xf(x)dx where

01 f(x)dx = A, is known.

Also, its is know that when x =1, f(x) =0 and when x =0, f(x) =1.

Can we get a solution of I in terms of A?

I have tried going the integration by-parts route which did not lead to any success. Any help is much appreciated.
 
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Without more details about f(x), this is impossible. It is easy to find some examples where the constraints are met, A is the same but I differs.
 
Unfortunately, f (x) is a transcendental function

f(x) = a - be[px+qf(x)] -c[px+qf(x)]

where, a, b, c, p and q are all constants. I don't think this helps?
 
I don't know if that helps, but it is at least some hope to make it possible (I just don't know how). Without knowing anything about f(x) it would be completely impossible.
 
It's going to be annoying, but if ##f## has an expansion as a power series, then you might be able to find an approximation to ##I##. I really doubt a analytic solution exists.
 
micromass said:
Wolfram alpha gives the following (very ugly) form of ##f(x)##: http://www.wolframalpha.com/input/?i=x+=+a+-+b*e^(p*r+q*x)+-c*(p*r+q*x)

(just replace ##x## by ##f(x)## and ##r## by ##x##). So yes, it's not going to be pretty.

wolframalpha gives the result in terms of Lambert's W function which I have already looked at. Looking for another route...
 
I am wondering if I can use rotation of coordinates to solve this integral, like here, Example D.9? Looking for some direction...
 
try this way, set u=x, dv=f(x)dx, you get du=dx and v=∫f(x)dx
then I=Ax-∫∫f(x)dxdx. Can you solve this?
 
  • #10
csleong said:
try this way,...Can you solve this?

Nope, not possible with what I know.
 

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