High order differential equations: undetermined coefficients

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SUMMARY

The discussion focuses on solving the third-order differential equation y''' - y' = te^(-t) + 2cos(t) using the method of undetermined coefficients. The correct form for the particular solution yp(t) is identified as t(At + B)e^(-t) + Ccos(t) + Dsin(t), which includes an additional t factor due to the presence of the term te^(-t) in the right-hand side. Participants clarify the characteristic equation r^3 - r = 0, confirming the roots as 0, 1, and -1, which are essential for determining the complementary solution. The discussion emphasizes the importance of correctly identifying the form of yp(t) based on the non-homogeneous terms.

PREREQUISITES
  • Understanding of third-order differential equations
  • Familiarity with the method of undetermined coefficients
  • Knowledge of characteristic equations and their roots
  • Basic concepts of complementary and particular solutions in differential equations
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  • Study the method of undetermined coefficients in detail
  • Learn about complementary solutions for higher-order differential equations
  • Explore examples of third-order differential equations with non-homogeneous terms
  • Investigate the implications of different roots in characteristic equations
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Students and educators in mathematics, particularly those studying differential equations, as well as anyone seeking to deepen their understanding of the method of undetermined coefficients and its applications in solving complex equations.

dmoney123
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Homework Statement



If the method of undetermined coefficients is used to find a particular solution
yp (t) to the differential equation y'''-y'=te^(-t)+2cos(t) should
have the form: ?

Homework Equations

The Attempt at a Solution


LHS

r^3-r=0

roots= 0, 1

y_c(t)=c_1e^tRHS

te^(-t)+2cos(t)

(At+B)e^(-t)+Ccos(t)+Dsin(t)

correct answer given however is

t(At + B)e^(-t) + C cos(t) + D sin(t)

I don't know how that t in front got there.. It would make sense if my LHS gave e^-t. but i don't think it does.Thanks
 
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Hello dmoney,

Your LHS should be yc(t) = c1et + c2e(0*t) = c1et + c2

missed that ... c3e-t
 
Your characteristic equation is third order in r. How many roots are there?
 
SteamKing said:
Your characteristic equation is third order in r. How many roots are there?

right... 3 roots...

so r^3-r=0

r=0, r=1, and... r=-1

-1-(-1)=0

I always get stuck on the stupidest mistakes.

I really appreciate it! thanks
 

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