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MathGroup Archive 2005

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Re: Assigning (really dumb question)

  • To: mathgroup at smc.vnet.net
  • Subject: [mg61005] Re: Assigning (really dumb question)
  • From: Peter Pein <petsie at dordos.net>
  • Date: Fri, 7 Oct 2005 03:37:49 -0400 (EDT)
  • References: <di2ml8$8h0$1@smc.vnet.net>
  • Sender: owner-wri-mathgroup at wolfram.com

vjp2.at at at.BioStrategist.dot.dot.com schrieb:
> What the heck am I doing wrong? I've been away from Mathematica for a while.
> 
> I can't get it to evaluate the expressions at the solved values.
> 
> I feel so stupid.
> 
> r[q_] := 1800 q - .006 q^2
> c[q_] := 12100000 + 800 q + .004 q ^2
> 
> qa = Solve[ D[ r[q]-c[q] , q] == 0, q]
> r[q] /. q -> qa
> c[q] /. q -> qa
> r[q]-c[q] /. q -> qa
> D[r[q],q] /. q -> qa
> D[c[q],q] /. q -> qa
> 
> qm = Solve[ c[q]/q - D[c[q] , q] == 0, q] [[2]]
> r[qm] /. q -> qm
> c[qm] /. q -> qm
> r[qm]-c[qm] /. q -> qm
> D[r[q],q] /. q -> qm
> D[c[q],q] /. q -> qm
> 
> 
> (PS, a while back I was asking how to fix DOS 2.2.2 when it wasn't
> doing graphics and I was told it was no longer supported. Well, 2yrs
> later, dumbo me, realised I just had to cd into the math directory and
> all was fine.)
> 
> 
> 				- = -
>     Vasos-Peter John Panagiotopoulos II, Columbia'81+, Bio$trategist
> 	      BachMozart ReaganQuayle EvrytanoKastorian
>        http://ourworld.compuserve.com/homepages/vjp2/vasos.htm
>   ---{Nothing herein constitutes advice.  Everything fully disclaimed.}---
>    [Homeland Security means private firearms not lazy obstructive guards]
>  [Fooey on GIU,{MS,X}Windows 4 Bimbos] [Cigar smoke belongs in veg food group]
> 

Hello,

what do you expect, when you replace q by a List of a list of Rules as in
r[q] /. q -> {{Britannia \[Rule] theWaves}}?

Or when calling r with a list of list of rules as parameter an applying 
the list of rules to the result?


Try r[q]/.qa etc., but I would prefer

qa=First[q/.Solve[r'[q]==c'[q]]] and
qm=First@Select[q/.Solve[c[q]==q*c'[q]],#>0&].

Then you can evaluate r[qa], c'[qm] etc directly.

Peter


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