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Re: cannot solve *trivial* equation

  • To: mathgroup at smc.vnet.net
  • Subject: [mg34412] Re: cannot solve *trivial* equation
  • From: lzhao at ihw.com.cn (Instanton)
  • Date: Sat, 18 May 2002 03:51:36 -0400 (EDT)
  • References: <ac2n7c$3qr$1@smc.vnet.net>
  • Sender: owner-wri-mathgroup at wolfram.com

Hi,

What really astonished me is that you did get a solution in the first
Solve. The reason that you cannot get a solution for S is that
Mathematica assumes only S as a variable, and A and Q are regarded as
independent parameters. Under such conditions you cannot get a
solution by hand either ...

The correct way of solving the system of equations is

Solve[{A == S + Q, Q == 2*S}, {S,A}]

which yields

{S->Q/2, A->3Q/2}

Instanton


Marco Manfredini <marco at technoboredom.net> wrote in message news:<ac2n7c$3qr$1 at smc.vnet.net>...
> Hi,
> 
> I just tried a friend's Mathematica 4.0.0.0: 
> 
> Solve[{A == S + Q, Q == 2*S}, A]
> 
> => {A->3*S}
> 
> Solve[{A == S + Q, Q == 2*S}, S]
> 
> => {}
>  
> Can somebody explain this to me? (ie. "bug","you stupid") 
> 
> Marco


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