I want to get a particular solution to the differential

James Dale

James Dale

Answered question

2022-01-18

I want to get a particular solution to the differential equation
y+2y+2y=2excos(x)
and therefore I would like to 'complexify' the right hand side. This means that I want to write the right hand side as q(x)eαx with q(x) a polynomial. How is this possible?

Answer & Explanation

scomparve5j

scomparve5j

Beginner2022-01-19Added 38 answers

The point is that (for real x) 2excos(x) is the real part of 2exeix=2e(1+i)x. Find a particular solution of y+2y+2y=2e(1+i)x, and its real part is a solution of y+2y+2y=2excos(x)
Lakisha Archer

Lakisha Archer

Beginner2022-01-20Added 39 answers

As cosx=eix+eix2,2excosx=ex+ix+exix, but that is not of the requested form. Is it close enough?
alenahelenash

alenahelenash

Expert2022-01-24Added 556 answers

I know that this might be awfully late, but I just started learning about complexification this term and thought I would put up my solution-please excuse any incorrect language I might use as it's the idea I am trying to get across.
First we start off by defining a complex analogue to your function:
eq 1: z+2z+2z=2exeix
where z=RE(y)+iIM(y)
Basically, we can recover the original diffEq by extracting the real part of our complex diffEq. The next step is to use the method of undetermined coefficients to find a guess for what our particular complex solution might be. Guess:
eq 2: z=Aexeix
so that: z=Aexeix+iAexeix
and z=i2Aexeix
Plugging this into 1:
i2Aexeix+2(Aexeix+iAexeix)+2(Aexeix)=2exeix
We can simplify by removing the common factor of exeix:
A(4+4i)=2A=12+2i
Convert A to complex polar form:
A=24eiπ4
Plugging this into 2:
z=24eiπ4exeix
This can be simplified to eq 3:
z=24exei(xπ4)
Since our particular solution should be of the form cos(x), we take the real part of 3 and call that our particular x-solution:
x=RE(z)=24excos(xπ4)
Finally using our difference of cosine identity:
24ex(cos(x)22+sin(x)22)
2422ex(cos(x)+sin(x))
28ex(cos(x)+sin(x))
14ex(cos(x)+sin(x))

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