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Re: Solve problem

  • To: mathgroup at smc.vnet.net
  • Subject: [mg98036] Re: Solve problem
  • From: "m.g." <mg at michaelgamer.de>
  • Date: Sat, 28 Mar 2009 05:44:47 -0500 (EST)
  • References: <gqia5h$oj3$1@smc.vnet.net>

On 27 Mrz., 11:35, araldocampan... at gmail.com wrote:
> I have a Solve[...] which gives me solutions like
> {{b1 -> -2, a1 -> -1 }, {b1 -> 0.5, a1 -> 3}}
> or like
> {{a1 -> 1, b1 -> 3}}
>
> How can I get the positive value for a1, which is  3
> in the first case and  1  in the second one?
>
> Bruno

Hello Bruno,

  maybe you think of the following:

A system of equations  - yust as it came in my mind :-)

In[42]:= gls={2 x+3 y==2,7 x-5 y^2==3}
Out[42]= {2 x+3 y==2,7 x-5 y^2==3}

the solution

In[43]:= lsg=Solve[gls,{x,y}] // N
Out[43]= {{x->0.506033,y->0.329311}, {x->4.64397,y->-2.42931}}

extracting values

In[45]:= temp={x,y} /. lsg
Out[45]= {{0.506033,0.329311},{4.64397,-2.42931}}

Selecting positive solution:

In[49]:= Select[temp,#[[2]]>0&]
Out[49]= {{0.506033,0.329311}}

Greetings

Mike


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