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MathGroup Archive 2005

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Re: Assigning (really dumb question)

  • To: mathgroup at smc.vnet.net
  • Subject: [mg61009] Re: [mg60988] Assigning (really dumb question)
  • From: Igor Antonio <igora at wolf-ram.com>
  • Date: Fri, 7 Oct 2005 03:37:56 -0400 (EDT)
  • Organization: Wolfram Research, Inc.
  • References: <200510060808.EAA08545@smc.vnet.net>
  • Reply-to: igora at wolf-ram.com
  • Sender: owner-wri-mathgroup at wolfram.com

vjp2.at at at.BioStrategist.dot.dot.com wrote:
> 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
> 
> 

Vasos,

First, do a First[] of the output of Solve since there's only one rule.

The problem with your code is that the placement of ReplaceAll is incorrect. 
It's replacing the "q" inside r[q] before it evaluates r[q].  Here's one way to 
do it:

In[63]:=
r[q_] := 1800*q - 0.006*q^2
c[q_] := 12100000 + 800*q + 0.004*q^2

In[65]:=
qa = First[Solve[D[r[q] - c[q], q] == 0, q]]

Out[65]=
{q -> 50000.}

In[66]:=
r[a]

Out[66]=
1800*a - 0.006*a^2

In[67]:=
r[q /. qa]

Out[67]=
7.5*^7




> 
> 				- = -
>     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]


-- 


Igor C. Antonio
Wolfram Research, Inc.
http://www.wolfram.com

To email me personally, remove the dash.


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