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MathGroup Archive 2000

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Re: A strange bug in Solve

  • To: mathgroup at smc.vnet.net
  • Subject: [mg24423] Re: [mg24354] A strange bug in Solve
  • From: "Kevin J. McCann" <kevinmccann at home.com>
  • Date: Tue, 18 Jul 2000 00:58:21 -0400 (EDT)
  • References: <8kjccd$dov@smc.vnet.net>
  • Sender: owner-wri-mathgroup at wolfram.com

While using Simplify is all well and good, its use presupposes that you know
what the answer is or that there is an answer. I have often seen posts on
similar types of problems which suggest that if you just do this, this, and
this to your problem then Mathematica will find the answer. This is NOT
satisfactory. If Mathematica needs to simplify the input before it executes Solve,
then it should do it. One should not be expected to know the answer before
asking Mathematica for it.

Kevin

<BobHanlon at aol.com> wrote in message news:8kjccd$dov at smc.vnet.net...
>
> In a message dated 7/9/2000 5:36:52 AM, andrzej at tuins.ac.jp writes:
>
> >I have long ago learned to be careful when making claims about kernel
bugs
> >in Mathematica, but this time I am pretty sure I have found a fairly
serious
> >one, even though it looks rather strange.
> >
> >I asked Mathematica to solve a system of two trigonometric equations:
> >
> >In[1]:=
> >eqns = {2*Cos[2*t]*Cos[u/2] + Cos[t]*Sin[u/2] == 0,
> >    (Cos[u/2]*Sin[t])/2 - (Sin[2*t]*Sin[u/2])/2 == 0};
> >
> >To my surprise Solve returned the empty list (I knew these equations do
> >have
> >solutions for geometric reasons):
> >
>
> Solve doesn't get confused if you simplify the expression:
>
> eqns = {2*Cos[2*t]*Cos[u/2] + Cos[t]*Sin[u/2] ==
>         0, (Cos[u/2]*Sin[t])/2 - (Sin[2*t]*Sin[u/2])/2 == 0};
>
> Solve[eqns // Simplify, {t, u}]
>
> "Solve::ifun : Inverse functions are being used by Solve, so some
solutions
> may not be found."
>
> {{u -> 2*ArcCos[-(1/Sqrt[5])], t -> 0},
>   {u -> -2*ArcCos[1/Sqrt[5]], t -> 0}}
>
> eqns /. %
>
> {{True, True}, {True, True}}
>
>
> Bob Hanlon
>



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