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Re: Assigning (really dumb question)

• To: mathgroup at smc.vnet.net
• Subject: [mg61016] Re: Assigning (really dumb question)
• From: Bill Rowe <readnewsciv at earthlink.net>
• Date: Fri, 7 Oct 2005 03:38:07 -0400 (EDT)
• Sender: owner-wri-mathgroup at wolfram.com

On 10/6/05 at 4:08 AM, 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.

>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

You've asked Mathematica to substitute the replacement rule qa for the variable q after evaluating r[q]. Since Solve returns a list of replacement rules you want to do

r[q]/.qa
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