# Real and imaginary solutions to eigenvector problem

So i have solved a system of linear ODEs problem given the matrix

$$\begin{bmatrix}-1 & -2\\5 & -3\end{bmatrix}$$ and initial conditions

y $$(0)$$= $$\begin{bmatrix}2\\1\end{bmatrix}$$

but dont know exactly how to find the real and imaginary solutions to the following solutions I calculated:

$$y_1$$ = $$\begin{bmatrix} \frac{1}{5} + \frac{3i}{5}\\1\end{bmatrix} e^ {-2+3i}$$ , and

$$y_2$$ = $$\begin{bmatrix} \frac{1}{5} - \frac{3i}{5}\\1\end{bmatrix} e^ {-2-3i}$$

From here, i tried applying Euler's formula $$e^{i \theta}$$ = $$cos \theta + isin \theta$$ to obtain:

$$Re \begin{bmatrix} -2cos \theta \\ 2sin \theta \end{bmatrix}$$ and $$Im \begin{bmatrix} 3sin \theta \\ 3 cos \theta \end{bmatrix}$$

Resulting in general solution $$y_{general} = C_1 \begin{bmatrix} -2cos \theta \\ 2sin \theta \end{bmatrix} + C_2 \begin{bmatrix} 3sin \theta \\ 3 cos \theta \end{bmatrix}$$

Im not sure about the way to obtain the real and imaginary parts of the solution but this was my attempt. Any help is greatly appreciated!

## 2 Answers

We have

$$e^{\lambda_1 t}v_1 = e^{(-2+3i)t}\begin{bmatrix}1+3i\\5\end{bmatrix} = e^{-2t}e^{3it}\begin{bmatrix}1+3i\\5\end{bmatrix} = e^{-2t}(\cos 3t + i \sin 3t)\begin{bmatrix}1+3i\\5\end{bmatrix} = \begin{bmatrix} e^{-2t}(\cos 3t -3 \sin 3 t) + i (3 \cos 3t +\sin 3 t)\\ e^{-2t}(5\cos3 t + 5i\sin 3t) \end{bmatrix}$$

Because the real and imaginary parts are both independent solutions, we can now write

$$X(t) = \begin{bmatrix}x(t)\\ y(t) \end{bmatrix} = c_1 e^{-2t}\begin{bmatrix}\cos 3t -3 \sin 3 t \\ 5\cos 3t \end{bmatrix} + c_2 e^{-2t}\begin{bmatrix}3 \cos 3t+\sin 3t\\ 5\sin 3t \end{bmatrix}$$

Use the ICs to find $$c_1$$ and $$c_2$$.

The final solution is

$$X(t) = \begin{bmatrix}x(t)\\ y(t) \end{bmatrix} = e^{-2t}\begin{bmatrix}2\cos 3t \\ \cos 3t + 3 \sin 3t \end{bmatrix}$$

• Thank you so much!! – lohboys May 16 at 22:18

Hint: $$\tilde{y_{1}}=\frac{y_{1}+y_{2}}{2}, \tilde{y_{2}}=\frac{y_{1}-y_{2}}{2i}$$

• i've tried working it out using this hint to no success. I don't obtain anything which looks remotely like a possible answer :'(. Could you perhaps go through using this hint? – lohboys May 16 at 1:39