Let me try to be more explicit.
Here is an expression: diff(f(x),x).
Maxima returns it unevaluated, diff(f(x),x).
substitute 0.8 for x. You get the expression
diff(f(0.8), 0.8)
This makes no sense, and Maxima objects.
If you mean to evaluate an expression including derivatives, at a
point, you could try
at(diff(f(x),x), x=0.8), which has the advantage of making sense.
On 10/5/11 10:17 AM, Evan Cooch wrote:
> I don't really see the problem. I impicitiy differentiate the
> charpoly, generating a function (i.e., the derivative). Imply want to
> evaluate the function at a particular point.
>
> In Maple, this is trivial:
>
> with(linalg):
> mat:=Matrix([[0,s_a*m_2,s_a*m_3],[s_o,0,0],[0,s_a.s_a]]);
> cp:=charpoly(mat,lambda);
> implicitdiff(cp,lambda,s_a);
> subs(s_a=0.8,s_o=0.4,m_2=1.2,m_3=1.4,lambda=0.849,%);
>
> yielding 0.834
>
> (can do the same thing in Mathematica, and Gauss...)
>
> Basically, all I (and students) need to be able to do is implicitly
> differentiate a function, and then evaluate that derivative at a point
> specific by known values of the variables in the derivative.
>
> So, how does one do this in Maxima?
>
>
> On 10/5/2011 12:50 PM, Richard Fateman wrote:
>> On 10/5/2011 9:31 AM, Evan Cooch wrote:...
>> ..
>>
>>> I think you should take rhs(part(sol,1)). and do the substitution.
>>
>> substituting 0.8 for x in d/dx ... makes no sense.
>>