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We have that system of differential equations: $$ \left\{ \begin{array}{ll} x'=-x \\ y'=-2y \\ \end{array} \right. $$ I have to solve that system but I only know the method of derive first equation and substitute in the second and get a second order differential equation which I know to solve. But it seems that that method doesn't work here. How can I solve it? I'm beginner at this kind of exercises. Thanks!

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  • $\begingroup$ Actually the method you are using is more complicated than what you need here! Can you think of any function whose derivative would be minus itself? ($f' = -f$) $\endgroup$ – Milloupe Jun 17 at 9:35
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These are two unrelated DE's and you have to solve them independently. The answer is $x(t)=ce^{-t}$ and $y=de^{-2t}$ where $c$ and $d$ are constants.

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This system is just a collection of 2 uncoupled ODEs, and so can be solved separately, just as we would a simple 1st order ODE.

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