       Re: Assigning (really dumb question)

• To: mathgroup at smc.vnet.net
• Subject: [mg60995] Re: [mg60988] Assigning (really dumb question)
• From: Bob Hanlon <hanlonr at cox.net>
• Date: Fri, 7 Oct 2005 03:37:40 -0400 (EDT)
• Sender: owner-wri-mathgroup at wolfram.com

```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]/.qa
c[q]/.qa
r[q]-c[q]/.qa
D[r[q],q]/.qa
D[c[q],q]/.qa

{q -> 50000.}

7.5*^7

6.21*^7

1.29*^7

1200.

1200.

qm=Solve[c[q]/q-D[c[q],q]==0,q][]
r[q]/.qm
c[q]/.qm
r[q]-c[q]/.qm
D[r[q],q]/.qm
D[c[q],q]/.qm

{q -> 55000.}

8.085*^7

6.82*^7

1.265*^7

1140.

1240.

Bob Hanlon

>
> From: vjp2.at at at.BioStrategist.dot.dot.com
To: mathgroup at smc.vnet.net
> Date: 2005/10/06 Thu AM 04:08:30 EDT
> Subject: [mg60995] [mg60988] Assigning (really dumb question)
>
> 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
>
>
> (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.)
>

Bob Hanlon
Chantilly, VA

```

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