Find the complementary function

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SUMMARY

The discussion focuses on solving the differential equation d²y/dx² - 2dy/dx - 3y = x by finding the complementary function, particular integral, and general solution. The complementary function is correctly identified as y = Ae^(3x) + Be^(-x), derived from the characteristic equation m² - 2m - 3 = 0. The next step involves determining a particular solution, which is independent of the complementary solution and typically involves intelligent guesswork, often using polynomial forms.

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doroulla
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the question is to
find the complementary function, particular integral and general solution of:
d2y/dx2 - 2dy/dx - 3y= x

i did: m^2 -2m -3 =0
i get (m-3)(m+1)=0
so i have y=Ae^3 + Be^-1
i don't know how to continue from here.. Is this the complementary function?
 
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hi doroulla! welcome to pf! :smile:

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doroulla said:
… so i have y=Ae^3 + Be^-1
i don't know how to continue from here.. Is this the complementary function?

the general solution to the complementary equation is y = Ae3x + Be-x

if y1 and y2 are any two solutions to the original equation, then y1 - y2 is a solution to the complementary equation

now you need to find one particular solution

the particular solution depends only on the RHS of the original equation, and usually has nothing to do with the LHS (so the particular solution usually has nothing to do with the complementary solution)

this is mostly intelligent guesswork

in this case, try a polynomial :smile:
 

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