Differential Equation dy/dx + yf'(x) = f(x).f'(x)

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Homework Help Overview

The discussion revolves around solving the linear differential equation dy/dx + yf'(x) = f(x)f'(x), where f(x) is a specified function of x. Participants are exploring the validity of a proposed solution and its implications.

Discussion Character

  • Exploratory, Problem interpretation, Assumption checking

Approaches and Questions Raised

  • Participants discuss the proposed solution y = f(x) - 1 and question its correctness, particularly in relation to differentiating it back to the original equation. There is also mention of the constant of integration and its significance in the solution.

Discussion Status

The discussion is active, with participants providing feedback on the proposed solution and clarifying misunderstandings. Some guidance has been offered regarding the inclusion of the constant of integration, and there is acknowledgment of a mistake made by one participant.

Contextual Notes

There is an emphasis on ensuring that the solution accounts for the constant of integration, which is a common requirement in differential equations. Participants are also navigating the implications of their interpretations of the original equation.

zorro
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Homework Statement



Solve dy/dx + yf'(x) = f(x).f'(x) where f(x) is a given function of x.


The Attempt at a Solution



This is a linear differential equation where I.F. = e^f(x)
On solving, I got the answer as
y = f(x) - 1 which doesnot make sense as differentiating it doesnot give back the equation in question. Where am I wrong?
 
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y=f(x)-1 is a solution. Put it into your original ODE and show you get an identity. It's not the most general solution though. You should probably keep a constant of integration.
 
Hi Abdul! :smile:
Abdul Quadeer said:
Solve dy/dx + yf'(x) = f(x).f'(x) where f(x) is a given function of x.

On solving, I got the answer as
y = f(x) - 1 which doesnot make sense as differentiating it doesnot give back the equation in question. Where am I wrong?

You're wrong about being wrong.

d(f(x) - 1)/dx + (f(x) - 1)f'(x) = f(x).f'(x)

(but you have left out the constant of integration :redface:)

EDIT: Dick beat me to it! :biggrin:
 
Exactly at 5:47 P.M.!

Ahh I made a very silly mistake. Thanks to both for your help :smile:
 

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