try
ratsubst(r^2,a^2+b^2+c^2, expression)
> -----Original Message-----
> From: maxima-bounces at math.utexas.edu
> [mailto:maxima-bounces at math.utexas.edu] On Behalf Of Richard Owlett
> Sent: Monday, June 09, 2008 7:30 AM
> To: Maxima List
> Subject: Forcing substitution
>
> I am new to Maxima doing two things - in order of importance
> 1. Review college algebra I had more than 40 years ago.
> It's scratch pad that is more legible than my handwriting and
> easier to save.
> 2. "Solve" a system of equations that are quadratic in x,y,z and
> linear in t. Fortunately all x^2, y^2, and z^2 terms cancel ;)
>
> The question.
>
> I get to a point of a simplified set of equations which contain
> subexpressions such as
> a^2 + b^2 + c^2
>
> I, having defined the problem, recognize that
> r^2 = a^2 + b^2 + c^2
>
> Can I have have Maxima recognize a^2 + b^2 + c^2 and substitute r^2 ?
> It may appear as -a^2 - b^2 - c^2 , for which I would like to
> substitute
> -r^2 .
> _______________________________________________
> Maxima mailing list
> Maxima at math.utexas.edu
> http://www.math.utexas.edu/mailman/listinfo/maxima
>