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Re: Solve inconsistant actions?

  • To: mathgroup at smc.vnet.net
  • Subject: [mg87236] Re: Solve inconsistant actions?
  • From: Jean-Marc Gulliet <jeanmarc.gulliet at gmail.com>
  • Date: Sat, 5 Apr 2008 04:21:54 -0500 (EST)
  • Organization: The Open University, Milton Keynes, UK
  • References: <001e01c895c3$23556720$6501a8c0@MLHC> <47F57015.8090002@gmail.com> <ft4n9s$42q$1@smc.vnet.net>

mhicks wrote:

> Can someone enlighten me as to what discriminators Solve uses in 
> providing some solutions while disdaining others in the attached simple 
> code?
>  
> Is it an operator or Mathematica bug? I am running 6.0.1 under win xp.
> 
> 
> newrent = Table[125000 \[ExponentialE]^(r 14), {r, .01, .04, .005}] // N
> 
> Table[Solve[newrent[[i]]/rate == 1894000 , rate], {i, 1, 7}]
> 
> Table[Solve[newrent[[i]]/rate == 1894000. , rate], {i, 1, 7}]
> 
> Table[Reduce[newrent[[i]]/rate == 1894000 , rate], {i, 1, 7}]
> 
> Table[Solve[newrent[[i]] == 1894000 rate, rate], {i, 1, 7}]

Keep in mind that *Solve* is a symbolic solver and it usually works 
better when fed with exact numbers, as you can see below. (Sorry not to 
be able to comment more on the internal algorithm of *Solve*.)

newrent = Table[125000 E^(r 14), {r, 1/100, 4/100, 5/1000}]

rate /. N@
    Table[Solve[newrent[[i]]/rate == 1894000, rate], {i, 1,
      7}] // Flatten

rate /. Table[
    Solve[newrent[[i]]/rate == 1894000., rate], {i, 1, 7}] // Flatten

rate /. Table[
    NSolve[newrent[[i]]/rate == 1894000, rate], {i, 1, 7}] // Flatten

rate /. Table[
    Solve[newrent[[i]]/rate == 1894000., rate], {i, 1, 7}] // Flatten

rate /. ToRules /@
   N@Table[Reduce[newrent[[i]]/rate == 1894000, rate], {i, 1, 7}]

rate /. N@
    Table[Solve[newrent[[i]] == 1894000 rate, rate], {i, 1,
      7}] // Flatten

  {125000 E^(7/50), 125000 E^(21/100), 125000 E^(7/25),
  125000 E^(7/20), 125000 E^(21/50), 125000 E^(49/100),
  125000 E^(14/25)}

{0.0759156, 0.0814201, 0.0873238, 0.0936555, 0.100446, \
0.107729, 0.115541}

{0.0759156, 0.0814201, 0.0873238, 0.0936555, 0.100446, \
0.107729, 0.115541}

{0.0759156, 0.0814201, 0.0873238, 0.0936555, 0.100446, \
0.107729, 0.115541}

{0.0759156, 0.0814201, 0.0873238, 0.0936555, 0.100446, \
0.107729, 0.115541}

{0.0759156, 0.0814201, 0.0873238, 0.0936555, 0.100446, \
0.107729, 0.115541}

{0.0759156, 0.0814201, 0.0873238, 0.0936555, 0.100446, \
0.107729, 0.115541}

HTH,
-- Jean-Marc


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