Undetermined Coefficient with repeated roots

In summary, The problem is to find the particular solution (yp) for the given differential equation, y" + 2y' - 3y = x^2*e^x. The roots of the characteristic equation are y1 = -3 and y2 = 1, which lead to the complementary solution of e^x. The student is unsure of how to set up the particular solution, but suggests two possible options: yp = x * (x^2*A*e^x) or x * (Ax^2 + Bx + C)e^x. They also mention another setup for a problem involving a cosine term.
  • #1
Ian88
2
0

Homework Statement



The problem is: y" + 2y' - 3y = x^2*e^x

Homework Equations





The Attempt at a Solution



I know the roots are y1 = -3 and y2 = 1, becoming e^x.

I'm not sure how to set my yp up though with the repeating e^x.

My ideas are yp = x * (x^2*A*e^x) or x * (Ax^2 + Bx + C)e^x.

I was also curious on the setup of a problem where yp = P(x)*e^ax* cos(bx).
 
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  • #2
since '1' is a root, the PI for ex is xex, and PI for x2 is Ax2+Bx+C, so put those 2 together

yp=xex(Ax2+Bx+C)
 

FAQ: Undetermined Coefficient with repeated roots

What is the "Undetermined Coefficient with repeated roots" method?

The "Undetermined Coefficient with repeated roots" method is a technique used in solving differential equations with repeated roots. It involves assuming a particular solution of the form Ax^n e^(rx) where n is the multiplicity of the repeated root and A is a constant to be determined.

When is the "Undetermined Coefficient with repeated roots" method typically used?

This method is typically used when solving linear differential equations with constant coefficients, where the characteristic equation has repeated roots.

What are the steps involved in using the "Undetermined Coefficient with repeated roots" method?

The steps involved in using this method are:1. Find the characteristic equation and determine the roots.2. If there are repeated roots, determine the multiplicity.3. Assume a particular solution of the form Ax^n e^(rx).4. Substitute this solution into the original differential equation and solve for A.5. Add the particular solution to the complementary solution to get the general solution.

What are some examples of differential equations where the "Undetermined Coefficient with repeated roots" method can be applied?

Some examples of differential equations where this method can be applied are:1. y'' + 6y' + 9y = 2e^(-3x)2. y'' + 4y' + 4y = 3xe^(-2x)3. y'' + 8y' + 16y = 4x^2 e^(-4x)

What are some limitations of the "Undetermined Coefficient with repeated roots" method?

Some limitations of this method include:1. It can only be used for linear differential equations with constant coefficients.2. It can only be used for repeated roots with a multiplicity of 1.3. It may not work for more complex differential equations with non-constant coefficients.

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