# Question:How do I get a derivative from a system of ODEs

## Question:How do I get a derivative from a system of ODEs

I have a system with unknown functions: R(x), mu(x), Y(x). I need to find them AND also mu'(x), Y'(x), Y''(x). How do I write it in the code?

I tried something like this:

```F := dsolve({cond, sys}, [R(x), mu(x), Y(x)], numeric, output = listprocedure);
Y1 := eval (Y(x), F);
Y2 := x -> diff(Y1(x), x);```

It works, but does it work correctly? Y1 is a massive of numbers, is it correct to write "a derivative from a massive of numbers"?

I also tried this:

```F := dsolve({cond, sys}, [R(x), mu(x), Y(x), diff(mu(x),x), diff(Y(x),x), diff(Y(x),x\$2)], numeric, output = listprocedure);
Y1 := eval (Y(x), F);
Y2 := eval (diff(Y(x),x), F);```

But it doesn't work at all and gives me an error on F:

Error, (in dsolve/numeric/process_input) dependent variables must be functions of a single unknown, the independent variable. Got [diff(Y(x), x), diff(diff(Y(x), x), x), diff(mu(x), x)]

How should I really do it?
3.1.mw
3.2.mw

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