Solve the Differential Equation y''−4y'+4y=2e^(2x)?

Drew Williamson

Drew Williamson

Answered question

2022-09-29

Solve the Differential Equation y - 4 y + 4 y = 2 e 2 x ?

Answer & Explanation

goffaerothMotw1

goffaerothMotw1

Beginner2022-09-30Added 10 answers

Method of Undetermined Coefficients

Start with the homogeneous equation and the complementary solution :

y - 4 y + 4 y = 0

This has characteristic equation:

λ 2 - 4 λ + 4 = 0 ( λ - 2 ) 2 = 0

Repeated roots mean that, in lieu of the usual solution y c = α e λ 1 x + β e λ 2 x , we look here for a solution in the form:

y c = e 2 x ( α x + β )

And because the non-homogeneous equation already has a e 2 x term, we must look at a particular solution in the form:

y p = γ x 2 e 2 x
y = 2 γ x e 2 x + 2 γ x 2 e 2 x
y = 2 γ e 2 x + 8 γ x e 2 x + 4 γ x 2 e 2 x

Putting these into the equation:

2 γ e 2 x + 8 γ x e 2 x + 4 γ x 2 e 2 x - 4 ( 2 γ x e 2 x + 2 γ x 2 e 2 x ) + 4 γ x 2 e 2 x = 2 e 2 x

γ = 1 and y p = x 2 e 2 x

The general solution is: y g = y c + y p

y g = e 2 x ( α x + β ) + x 2 e 2 x

= e 2 x ( x 2 + α x + β )

Now applying the IV's:

y ( 0 ) = 1 β = 1 y g = e 2 x ( x 2 + α x + 1 )

y = 2 e 2 x ( x 2 + α x + 1 ) + e 2 x ( 2 x + α )

= e 2 x ( 2 x 2 + ( 2 α + 2 ) x + ( 2 + α ) )

And from the second IV, y ( 0 ) = 4 α = 2

y g = e 2 x ( x 2 + 2 x + 1 )

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