A question on undetermined coefficients

In summary, the question asks why the particular solution (yp) for the differential equation D^3 + 4D = 12t + 8 is yp = At^3 + Bt^2 + Ct instead of yp = At^2 + Bt + C. The reason is that the product of the differential operator (D^3 + 4D) and the proposed solution must result in a polynomial of the same degree as the right-hand side of the equation. In this case, the proposed solution of yp=At^3 + Bt^2 + Ct results in a polynomial of degree 3, while yp=At^2 + Bt + C would only result in a polynomial of degree 2.
  • #1
shemer77
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Homework Statement


http://gyazo.com/6c440aa92106f729639c91f6d59dcd89


The Attempt at a Solution


My question is why is yp = At^3+Bt^2 +Ct. The reason I ask that is because is see t^2+2t so why wouldn't it be yp=At^2 +Bt +c?
 
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  • #2
shemer77 said:

Homework Statement


http://gyazo.com/6c440aa92106f729639c91f6d59dcd89

The Attempt at a Solution


My question is why is yp = At^3+Bt^2 +Ct. The reason I ask that is because is see t^2+2t so why wouldn't it be yp=At^2 +Bt +c?
attachment.php?attachmentid=48917&stc=1&d=1341594524.png


Because (D3 + 4D)( At3+Bt2 +Ct) = 12At2 + 8Bt + 6A + 4C .

Whereas, (D3 + 4D)( At2+Bt+C) = 8At + 4B, which is not a quadratic.

.
 

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Related to A question on undetermined coefficients

1. What is the concept of undetermined coefficients?

The concept of undetermined coefficients is a method used in solving differential equations, specifically in finding a particular solution. It involves assuming a general form for the particular solution and then using it to determine the coefficients that will satisfy the given differential equation.

2. When is the undetermined coefficients method used?

The undetermined coefficients method is typically used when the homogeneous solution of a differential equation is known, and the non-homogeneous part of the equation can be expressed as a sum of products of known functions and their derivatives.

3. How is the particular solution found using undetermined coefficients?

To find the particular solution using undetermined coefficients, we first assume a general form for the particular solution based on the non-homogeneous part of the equation. We then substitute this into the differential equation and solve for the coefficients using algebraic methods.

4. Can the undetermined coefficients method be used for all types of differential equations?

No, the undetermined coefficients method can only be used for linear differential equations with constant coefficients. It cannot be used for nonlinear or variable coefficient equations.

5. Are there any limitations to the undetermined coefficients method?

One limitation of the undetermined coefficients method is that it only works for certain types of non-homogeneous terms. If the non-homogeneous term cannot be expressed as a sum of products of known functions and their derivatives, then this method cannot be used.

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