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MathGroup Archive 2005

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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)
  • Reply-to: hanlonr at cox.net
  • 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][[1]]
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][[2]]
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] [[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.)
> 


Bob Hanlon
Chantilly, VA


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