A question on undetermined coefficients

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SUMMARY

The discussion centers on the application of the method of undetermined coefficients in solving differential equations. The user questions why the particular solution is expressed as yp = At^3 + Bt^2 + Ct instead of yp = At^2 + Bt + C, given the non-homogeneous term t^2 + 2t. The resolution lies in the differentiation process, where applying the operator (D^3 + 4D) to the cubic form yields the correct order of terms, while the quadratic form does not satisfy the equation. This highlights the importance of matching the degree of the polynomial in the particular solution to the non-homogeneous term.

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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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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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