# Thread:

1. ## Operator method help

im trying to solve a system of differential equations of (x''(t ) + 5(x(t )) - 2y(t ) = 0
and, -2*x(t ) + y''(t ) +2*y(t ) = 0

but I keep getting the wrong answer..

2. Originally Posted by Mist
im trying to solve a system of differential equations of (x''(t ) + 5(x(t )) - 2y(t ) = 0
and, -2*x(t ) + y''(t ) +2*y(t ) = 0

but I keep getting the wrong answer..
Here's an outline of the solution. Write the equations as

$x'' = -5x+2y$
$y'' = 2x-2y,$

or in vector form $\begin{bmatrix}x''\\y''\end{bmatrix} = \begin{bmatrix}-5&2\\2&-2\end{bmatrix}\begin{bmatrix}x\\y\end{bmatrix}$.

Now diagonalise that matrix $A = \begin{bmatrix}-5&2\\2&-2\end{bmatrix}$. You should find that the eigenvalues are –1 and –6, with corresponding eigenvectors $\begin{bmatrix}1\\2\end{bmatrix}$ and $\begin{bmatrix}-2\\1\end{bmatrix}$. Let P be the matrix having those eigenvectors as columns, and let D be the diagonal matrix with the eigenvalues –1 and –6 as its diagonal elements. Then $A = PDP^{-1}$ and so $P^{-1}\begin{bmatrix}x''\\y''\end{bmatrix} = DP^{-1}\begin{bmatrix}x\\y\end{bmatrix}$.

Let $\begin{bmatrix}u\\v\end{bmatrix} = P^{-1}\begin{bmatrix}x\\y\end{bmatrix}$. Then $u'' = -u$ and $v'' = -6v$. Those are simple harmonic motion equations, with solutions $u = A\cos t + B\sin t$, $v = C\cos(\sqrt6t)+D\sin(\sqrt6t)$.

Finally, $\begin{bmatrix}x\\y\end{bmatrix} = P\begin{bmatrix}u\\v\end{bmatrix}$, giving the solution

$x(t) = A\cos t + B\sin t -2(C\cos(\sqrt6t)+D\sin(\sqrt6t)),$
$y(t) = 2(A\cos t + B\sin t) + C\cos(\sqrt6t)+D\sin(\sqrt6t).$
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