2nd ODE constant coeff, quick question

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SUMMARY

The discussion focuses on solving the second-order ordinary differential equation (ODE) given by \(\frac{d^2y}{dx^2}+\frac{dy}{dx}=3x^2+2x+1\). The complementary function (C.F.) is identified as \(y=C_1e^{-x}\), while the particular integral (P.I.) is proposed in the form \(y_{PI}=Ax^3+Bx^2+Cx+D\). The participant queries how to determine the constant \(D\), noting that differentiating \(y_{PI}\) results in its disappearance. It is concluded that \(D\) is redundant, as the constant function is defined by initial conditions rather than the forcing function.

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Homework Statement


Solve:
\frac{d^2y}{dx^2}+\frac{dy}{dx}=3x^2+2x+1

The Attempt at a Solution



Well the C.F. is y=C_1e^{-x}
the P.I. is of the form y_{PI}=Ax^3+Bx^2+Cx+D

I can find the values of A,B and C bu differentiating it and substituting it into the equation. But How would I find D since there is no 'y' in the ODE given and differentiating y_{PI} makes the constant disappear.
(Note: I can find the answer by integrating it w.r.t x and then using the integrating factor but I would like to know how to find it by adding the PI and CF together)
 
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rock.freak667 said:

Homework Statement


Solve:
\frac{d^2y}{dx^2}+\frac{dy}{dx}=3x^2+2x+1

The Attempt at a Solution



Well the C.F. is y=C_1e^{-x}
the P.I. is of the form y_{PI}=Ax^3+Bx^2+Cx+D

I can find the values of A,B and C bu differentiating it and substituting it into the equation. But How would I find D since there is no 'y' in the ODE given and differentiating y_{PI} makes the constant disappear.



(Note: I can find the answer by integrating it w.r.t x and then using the integrating factor but I would like to know how to find it by adding the PI and CF together)

The D is redundant since one of the solutions to the natural equation is a constant function.

Therefor the constant function is determined by the initial conditions rather then the forcing function.
 
Ah...thanks then,I thought I was really doing something wrong.
 

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