# [Maxima] Q: How to control rewriting

Stavros Macrakis macrakis at alum.mit.edu
Wed Apr 4 08:10:49 CDT 2012

```Not clear what you really want...

Maybe ratsubst (..., c/e, eq1)  solves your problem?

-s
On Apr 3, 2012 4:15 PM, "Enrique Perez-Terron" <enrio at online.no> wrote:

> Hello,
>
> I have an expression of the form
>
>          A*B*C*D
> EQ1: F = ---------
>             E
>
> where C/E has a particular simplification.
>
> I would like to have this expression rewritten on the form
>
>                C
> EQ2: F = A*B*D*---
>                E
>
> This would allow me to use, e.g.
>
>  substpart( simplify(piece), EQ2, 2, 4 )
>
> My question is how to produce EQ2 from EQ1, short of rewriting manually or
> copying and pasting.
> I am clumsy; when I do complex things manually I invariably do mistakes,
> forget a parenthesis or whatever, so I would like to have methods of doing
> this where I either get an error message or I get something that is
> mathematically equivalent to what I started with.
>
> Maybe
>
>  A: part(EQ1, 2,1,1)
>  B: part(EQ1, 2,1,2)
>  C: part(EQ1, 2,1,3)
>  D: part(EQ1, 2,1,4)
>  E: part(EQ1, 2,2)
>  F: part(EQ1, 1)
>
>  EQ2: F = A*B*D*(C/E)
>
>  EQ1 - EQ2
>
> would be an approach, where the last line is the control. (Assume that the
> letters A-E are actually not in use up to this point.)
>
> Now, this could work, but it is quite a bit of writing. I wish something
> more legible too.
>
> How can I do this?
>
> Thanks
> Enrique
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