How to Use the Method of Undetermined Coefficients for Differential Equations?

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SUMMARY

The discussion focuses on setting up the appropriate particular solution \( y_p \) for the differential equation \( y'' + 25y = -1x \cos(5x) \) using the Method of Undetermined Coefficients. The proposed form for the particular solution is \( y_p = (Ax^2 + Bx) \cos(5x) + (Cx^2 + Dx) \sin(5x) \) due to the presence of \( \cos(5x) \) and \( \sin(5x) \) as solutions to the homogeneous equation. This adjustment is necessary to account for the overlap with the homogeneous solution, ensuring the correct form is used for undetermined coefficients.

PREREQUISITES
  • Understanding of differential equations, specifically second-order linear equations.
  • Familiarity with the Method of Undetermined Coefficients.
  • Knowledge of homogeneous and particular solutions in differential equations.
  • Basic trigonometric functions and their derivatives.
NEXT STEPS
  • Study the Method of Undetermined Coefficients in detail.
  • Learn how to identify homogeneous and particular solutions in differential equations.
  • Practice setting up particular solutions for various forms of non-homogeneous terms.
  • Explore examples involving trigonometric functions in differential equations.
USEFUL FOR

Students and educators in mathematics, particularly those studying differential equations, as well as anyone looking to deepen their understanding of the Method of Undetermined Coefficients.

Tom McCurdy
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Homework Statement



Set up but do not solve for the appropriate particular solution yp for the differential equation

y''+25y=–1xcos(5x)

using the Method of Undetermined Coefficients (primes indicate derivatives with respect to x).

In your answer, give undetermined coefficients as A, B, etc.

Homework Equations


Possible Derivatives
  • sin(x)
  • cos(x)
  • xsin(x)
  • xcos(x)



The Attempt at a Solution



This seems like the solution should simply be

[tex]A*x*sin(5*x)+B*x*cos(5*x)+C*cos(5*x)+D*sin(5*x)[/tex]
 
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What solution are you talking about? The problem said "Set up but do not solve for the appropriate particular solution yp for the differential equation
y''+25y=–1xcos(5x)
using the Method of Undetermined Coefficients"

Normally, to get a solution of the form "x cos(5x)" you would use (Ax+B) cos(5x)+ (Cx+ D) sin(5x). However, in this case cos(5x) and sin(5x) are solutions to the homogeneous equation. Multiply what you would normally use by x: (Ax2+ Bx)cos(5x)+ (Cx2+ Dx)sin(5x).
 

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