Re: Assigning (really dumb question)

*To*: mathgroup at smc.vnet.net*Subject*: [mg61005] Re: Assigning (really dumb question)*From*: Peter Pein <petsie at dordos.net>*Date*: Fri, 7 Oct 2005 03:37:49 -0400 (EDT)*References*: <di2ml8$8h0$1@smc.vnet.net>*Sender*: owner-wri-mathgroup at wolfram.com

vjp2.at at at.BioStrategist.dot.dot.com schrieb: > 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.) > > > - = - > 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] > Hello, what do you expect, when you replace q by a List of a list of Rules as in r[q] /. q -> {{Britannia \[Rule] theWaves}}? Or when calling r with a list of list of rules as parameter an applying the list of rules to the result? Try r[q]/.qa etc., but I would prefer qa=First[q/.Solve[r'[q]==c'[q]]] and qm=First@Select[q/.Solve[c[q]==q*c'[q]],#>0&]. Then you can evaluate r[qa], c'[qm] etc directly. Peter