Simple Differential Equation (Ordinary Differential Equation)

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Discussion Overview

The discussion revolves around solving a first-order ordinary differential equation (ODE) of the form y' + 2y = 4(x + 1)². Participants explore methods for verifying solutions and approaches to finding the general solution, including the use of integrating factors and Bernoulli's equation.

Discussion Character

  • Technical explanation
  • Mathematical reasoning
  • Homework-related

Main Points Raised

  • Some participants identify the order of the ODE as first order.
  • There is a discussion on how to check if a particular function solves the ODE, with one participant suggesting substituting the function and its derivative into the equation.
  • Another participant proposes integrating the left-hand side of the ODE as a method to find the solution.
  • One participant mentions that verifying a solution is generally easier than finding it, implying a distinction between the two processes.
  • A later reply introduces the concept of using an integrating factor to solve the ODE, suggesting a specific form for g(x) and indicating that the resulting ODE is separable.
  • Another participant references their teacher's instruction to use Bernoulli's equation and notes that a constant term can be treated as C.

Areas of Agreement / Disagreement

Participants express various methods for approaching the problem, with no consensus on a single solution method. Disagreement exists on the best approach to take, with some favoring verification of solutions and others advocating for integration techniques or the use of Bernoulli's equation.

Contextual Notes

Some participants mention specific methods like integrating factors and Bernoulli's equation without fully resolving how these methods apply to the original ODE. There are also indications of missing steps in the verification process and assumptions about the nature of the solutions.

eskie
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Problem:
y'+2y=4(x+1)2 ----> y=5e-2x+2x2+2x+1

1. What the Order of the ODE?
It's 1st order

2 How do you check whether a particular function solves an equation?
If you solve y'+2y=4(x+1)2 and make it y=5e-2x+2x2+2x+1. I want the whole solution... thanks...
 
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Redo the whole thing:

1) What is the order of the ODE?

2) How do you check whether a particular function solves an equation?
 
eskie said:
Problem:
y'+2y=4(x+1)2 ----> y=5e-2x+2x2+2x+1

1. What the Order of the ODE?
It's 1st order
Yes, that is correct.

2 How do you check whether a particular function solves an equation?
If you solve y'+2y=4(x+1)2 and make it y=5e-2x+2x2+2x+1. I want the whole solution... thanks...
How would you determine whether x= 7 satisifies the equation [itex]x^9+ 9x^3- 2x^2+ 3x- 4= 0[/itex]? Not by actually solving the equation! Just put x= 7 into the equation and see if it makes the equation true or not.

Same thing here. You would NOT need to actually solve the equation. Just calculate the derivative of y: [itex]y' = -10e^{-2x}+ 4x+ 2[/itex] and put it and [itex]y= 5e^{-2x}+ 2x^2+ 2x+ 1[/itex] into the equation: [itex]y'+ 2y= -10e^{-2x}+ 4x+ 2+ 10e^{-2x}+ 4x^2+ 4x+ 2[/itex] is that equal to 4(x+ 1)?
 
oh i see... how about integrate the y' of y'+2y=4(x+1)2?
 
eskie said:
oh i see... how about integrate the y' of y'+2y=4(x+1)2?
That would be the way to find the solution, rather than verifying that a given function is the solution!

To find a solution is generally a lot harder to do than verifying that a function is a solution (or not).
 
oh... i see... i just to solve that equation just like this...
y'=4x
dy/dx=4x
dy=4xdx
y=2x2
 
eskie said:
oh... i see... i just to solve that equation just like this...
y'=4x
dy/dx=4x
dy=4xdx
Not quite!

In your original ODE, that approach won't help you much.

You need to use what we call an "integrating factor" here!

Let g(x)=e^{2x}y(x}

Then, we have:
[tex]\frac{dg}{dx}=e^{2x}(y'+2y)[/tex]

Note that the expression in the parenthesis is the left-hand side of your ODE, so that you may write:
[tex]\frac{dg}{dx}=4e^{2x}(x+1)^{2}[/tex]

THIS ODE is separable, and you can proceed to solve for g(x) first, nd then for y(x)..
 
oh i see... thanks anyway... :) our teacher said that we must use bernoulli's eq. to solve the eq... and the 5e-2x is constant.. therefore i can just change it to C
 

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