Differential Equations question

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Homework Help Overview

The discussion revolves around determining the values of m for which the function ∅(x) = xm is a solution to two differential equations: a) 3x²y'' + 11xy' - 3y = 0 and b) x²y'' - xy' - 5y = 0. Participants are exploring the process of substitution and differentiation in the context of differential equations.

Discussion Character

  • Exploratory, Mathematical reasoning, Assumption checking

Approaches and Questions Raised

  • The original poster attempts to substitute the function into the equations and differentiate it, but expresses uncertainty about the correctness of their approach and the next steps. Other participants question the differentiation process and suggest substituting derivatives back into the equations while considering the powers of x. There is also a focus on resolving mistakes in the differentiation process.

Discussion Status

The discussion has seen participants providing guidance on differentiation and substitution, with some expressing confusion over the handling of variables. The original poster acknowledges a mistake and appears to be working towards a resolution, indicating a productive direction in the conversation.

Contextual Notes

Participants are navigating through the complexities of differentiation and substitution in differential equations, with an emphasis on ensuring accuracy in their calculations. There is a recognition of the potential for simple errors to lead to significant confusion.

jawhnay
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Homework Statement


Determine for which values of m the function ∅(x) = xm is a solution to the given equation
a) 3x2y" + 11xy' -3y = 0
b) x2 y" - xy' - 5y = 0

The Attempt at a Solution


I tried approaching this problem by substituting ∅(x) into the question.
a) 3x2(xm)'' + 11x(xm)' - 3(xm) = 0
xm(3x2m2 + 11xm - 3) = 0
I don't know what to do after this step and I'm not sure this is the correct way to do this problem. I just tried doing this because I looked at a similar question and this is how it was approached in the solutions manual.
 
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Check your differentiation. What is d2/dx2(xm)?
 
y'= mxm-1 y''=(m-1)(m)xm-2 Are you asking me to substitute these into the given equation? I actually did that but what am I going to do with the x variable since I'm looking for what m is equal to?
 
Yes. Substitute them back in, but mind how the powers of x add up. What do you get?
 
3x2(mxm-1) + 11x(m-1)(m)xm-2 - 3(xm) = 0
3x2mxm-1 + 11x(m2-m)xm-2 - 3xm = 0
xm(3x2mx-1 + 11x(m2-m)x-2 - 3) = 0
m3x + 11x - 1(m2-m) - 3 = 0
m3x + m211x-1 - m11x-1 - 3 = 0
m(3x + m(11/x) - 11/x) = 3

I stopped right there since I wasn't really sure how I was going to get rid of the x...
 
Shouldn't it be 3x2((m-1)(m)xm-2) + 11x(mxm-1) - 3(xm) = 0?
 
oh my god... I can't believe I didn't catch that. Okay, let me try to do this again.
 
I finally got the answer. I can't believe one stupid mistake like that got me stuck on this problem. Thanks a lot for pointing out that mistake for me, alan!
 

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