I need to find a second order linear homogeneous equation

Edmund Adams

Edmund Adams

Answered question

2021-11-21

I need to find a second order linear homogeneous equation with constant coefficients that has the given function as a solution
Queston a) xe3x
Question b) e3xsinx

Answer & Explanation

Hiroko Cabezas

Hiroko Cabezas

Beginner2021-11-22Added 18 answers

Question A Let the answer to question a be of the form
ay+by+cy=0
Then we want a(3)23b+c=9a3b+c=0 (1)
And more over, since it is a repeated root we need
b2a=3
Substituting b=6a into (1) we get
9a18a+c=0⇒=9a
Thus if we let a be our free variable, all solutions to question a are of the form
ay+6ay+9ay=0
Question B For question b, you need the characteristic polynomial to have the roots 3±i. Thus the characteristic polynomial must be of the form
a(x[3+i])(x[3i])=ax26ax+10a
Thus solutions to question b are of the form
ay6ay+10ay=0
Note For your solution to question a you got y6y+9y=0 but going back and checking you should get +6y not 6y. Otherwise your solutions are of these forms with a=1 but a can be any real number. It is our free variable.
Unpled

Unpled

Beginner2021-11-23Added 23 answers

Hints For the first one, λ1=λ2=3 are roots of the chair. polynomial , so
χ(λ)=(λ(3))2=λ2+6λ+9
Which corresponds to the diff. eq.
y+6y+9y=0
Which has, as already found, the general solution
y=c1e3x+c2e3x
Now choose the values of y(0) and y(0) in such a way that c1=0 and c2=1. With y(0) we have
y(0)=c1
So we can set y(0)=0 to set c1=0. For y(0) we have
y(x)=ddx(c2xe3x)=c2e3x±3xc2e3x=c2e3x(13x)
Then
y(0)=c2
So we set y(0)=1 to get when we want.
For the second, you should know that if the solution λ to χ(λ) looks like
e3xsin(x)
Then deduce the characteristic polynomial from it, from which you can deduce the original differential equation.
Start by identifying λ1=3+i, deduce the complex conjugate pair as a2=3i, now set
χ(λ)=(λλ1)(λλ2)
To get
χ(λ)=λ26λ+10
Which corresponds to y6y+10y=0
Which has the general solution y(x)=c1e3xcos(x)+c2e3xsin(x)
Again, as above, choose y(0) and y'(0) in such a way that c1=0 and c2=1 to give the desired solution.
We see that y(0)=c1
So we need to set y(0)=0. But y(x) is a little bit harder t ocompute, I leave that to you.

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