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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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