My Problem is this given System of differential Equations: <mrow class="MJX-TeXAtom-ORD">

Marianna Stone

Marianna Stone

Answered question

2022-05-21

My Problem is this given System of differential Equations:
x ˙ = 8 x + 18 y
y ˙ = 3 x 7 y
I am looking for a gerenal solution.
My Approach was: i can see this is a System of linear and ordinary differential equations. Both are of first-order, because the highest derivative is the first. But now i am stuck, i have no idea how to solve it. A Transformation into a Matrix should lead to this expression:
y = ( 8 18 3 7 ) x
or is this correct:
x = ( 8 18 3 7 ) y  ?
But i don't know how to determine the solution, from this point on.

Answer & Explanation

Ueberbachge

Ueberbachge

Beginner2022-05-22Added 7 answers

I'm going to rename your variables. Instead of x and y, I will use x 1 and x 2 (respectively).
Now, let's look at the system:
{ x ˙ 1 = 8 x 1 + 18 x 2 x ˙ 2 = 3 x 1 7 x 2
To change this into matrix form, we rewrite as x ˙ = A x , where A is a matrix.
This looks like:
( x ˙ 1 x ˙ 2 ) x ˙ = ( 8 18 3 7 ) A ( x 1 x 2 ) x
To solve the system, we find the eigenvalues of the matrix. These are r 1 = 2 and r 2 = 1. Two corresponding eigenvectors are ξ 1 = ( 3 1 ) and ξ 2 = ( 2 1 ) , respectively.
We now plug these into the equation:
x = c 1 e r 1 t ξ 1 + c 2 e r 2 t ξ 2
This yields:
x = c 1 e 2 t ( 3 1 ) + c 2 e t ( 2 1 )
So, your individual solutions are:
x 1 = 3 c 1 e 2 t + 2 c 2 e t x 2 = c 1 e 2 t c 2 e t

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