Finding general solution of an ode using substitution

In summary, the conversation discusses the transformation u= x^αy and solving the differential equation y'' + (2/x)y' + 9y=0 using this transformation. The general solution is found to be y=(Acos3x+Be^-3x)/x, where A and B are constants. The conversation also mentions the possibility of using spherical Bessel functions for similar equations.
  • #1
honeypuffy
4
0

Homework Statement



By making the transformation u= x^αy where α is a constant to be found, find the general solution of[/B]

y'' + (2/x)y' + 9y=0

The Attempt at a Solution



I've worked out y,y',y'' and subbed them into get

x^-au'' + x^a-1(2-2a)u' + x^-a-2(x^2-a(a-1))u =0

but I don't know what to do from here.

Any help would be greatly appreciated.
 
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  • #2
Can you perhaps chose ##a## in some intelligent fashion to simplify your equation?
 
  • #3
yes sorry I forgot that bit I had worked out that a should be -1 and there's a typo it should be a(a+1)
which would simplify it down to

x^1 U'' + x^1U=0

then I had the thought to divide by x to get U''+U=0

but I got stuck here because I wasn't sure on what to do. Should I change back to y?
 
  • #4
No, you should solve the differential equation you obtained for u. Reinsering y would just give back the old differential equation.

Also, your original equation had a 9. Where did that go?
 
  • #5
OK so I would have U''+9U=0

with general solution of U=Ae^3x+Be^-3x

and U=y/x so y=(Ae^3x+be^-3x)/x
 
  • #6
No, this does not solve the differential equation.
 
  • #7
sorry U should be Acos3x+Bcos3x
 
  • #8
Correct. For equations of this type in general, you may want to have a look at spherical Bessel functions.
 

FAQ: Finding general solution of an ode using substitution

What is the concept of substitution in solving ODEs?

Substitution is a technique used in solving ordinary differential equations (ODEs) where a new variable is introduced to simplify the equation. This new variable is substituted into the original equation, making it easier to solve.

How does substitution help in finding the general solution of an ODE?

By introducing a new variable through substitution, the original ODE can be transformed into a simpler form that is easier to integrate and solve. This allows for the derivation of a general solution that satisfies the given ODE.

What are the common types of substitution used in solving ODEs?

The most common types of substitution used in solving ODEs are the trigonometric substitution, power substitution, and logarithmic substitution. These techniques are chosen based on the form of the ODE and the desired simplification.

Can substitution be applied to all types of ODEs?

No, substitution cannot be applied to all types of ODEs. It is most effective for first-order ODEs and some higher-order ODEs with separable variables or those that can be reduced to separable form. Other techniques, such as the method of undetermined coefficients or variation of parameters, may be used for different types of ODEs.

Are there any limitations or drawbacks to using substitution in solving ODEs?

While substitution can be a powerful tool in solving ODEs, it is not always the most efficient or effective method. In some cases, it may lead to more complex equations or require additional steps to reach a solution. It is important to consider the form of the ODE and the desired level of accuracy when determining the most appropriate method for solving an ODE.

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