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

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

Out=
{q -> 50000.}

In:=
r[a]

Out=
1800*a - 0.006*a^2

In:=
r[q /. qa]

Out=
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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