clue needed on using results of previous computations

Try z:rhs(x=-5*g/2).
plot(z, g, .....)

It is possible to make a function from z by...

   h(g):=''z

that is two single quotes in there.

I hope this fixes the hurdle!

RJF


Rich Drewes wrote:

> Hello,
> 
> I am making a dogged effort to use Maxima in a graduate math class,
> despite the bias toward Maple and Mathematica.  I have had some success
> but I am having great problems doing a couple of things that seem trivial
> in Maple.  I think this is because I just lack one crucial clue, and
> hopefully someone out there can help me out.
> 
> Here is one problem I am having.  Take a function, e.g.:
> 
> y=x*x + 5*g*x;
> 
> Say I want to plot x and g that maximizes y.  So, differentiate the
> function with respect to y, set equal to 0, and plot the x and g that
> satisfy that equation.  Sounds simple, and in Maple it is, but in Maxima I
> can't get past some conceptual hurdle.  Let's try this in Maxima:
> 
> eq1: y(x, g):=x*x + 5*g*x;
> 
> and that works.  We can differentiate with respect to x just fine:
> 
> diff(y(x, g), x);
> 
> which gives 2 x + 5 g as we expect.  So set this =0 and solve for x:
> 
> eq2:solve(diff(y(x, g), x) = 0, x);
> 
> This gives us a result of x=-5*g/2 as it should.  However, and this is
> where my conceptual problem arises, Maxima seems to treat that result
> as (forgive my terminology) an 'equation' rather than a 'function',
> and further manipulation is therefore difficult.  We can examine eq2:
> 
> eq2;
> 
> and that does spit back x=-5*g/2.  But how can we plot this?  How can
> we differentiate it further, with respect to g say?  For example,
> 
> plot2d(x, [g, 0, 10]);
> 
> fails, saying that the 'function ~a is undefined', whatever that is.
> Nor does:
> 
> plot2d(eq2, [g, 0, 10]);
> 
> work.  I can use ev() to evaluate this equation at some g, for example:
> 
> ev(eq2, g:5);
> 
> does what you would expect.  I could do an explicit redifinition by typing
> this in:
> 
> f(g):=-(5*g)/2;
> 
> and then this plot works:
> 
> plot2d(f(g), [g, 0, 10]);
> 
> But that's a pain in general.  I also can't differentiate this again:
> 
> diff(eq2, g);
> 
> since that returns the nonsensical "0=-5/2" rather than something
> symbolically reasonable like dx/dg=-5/2, where dx/dg can be referenced
> symbolically in another computation.  (Interestingly, if I do a
> "depends(f, g)" and then do the "diff(eq2, g)", I do get the result
> dx/dg=-5/2, but I still cannot reference the dx/dg result in other
> computations as a symbol . . .)
> 
> I guess the clearest statement of my conceptual problem is this:  whereas
> Maple's results seem to be treated in the same class as the stuff you type
> in directly, and these results are trivial to manipulate further and
> combine with other equations, in Maxima the results of operations seem to
> be treated as equations rather than functions, and they become difficult
> to manipulate further and I quickly get into the position of having to
> redefine functions explicitly by typing it back in as above, which is
> obviously a waste of time.
> 
> Does this make sense to anyone?
> 
> Thanks,
> Rich
> 
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