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Re: Another plea for help...

  • To: mathgroup at
  • Subject: [mg39171] Re: Another plea for help...
  • From: atelesforos at (Orestis Vantzos)
  • Date: Sun, 2 Feb 2003 01:13:16 -0500 (EST)
  • References: <b1f6h6$ara$>
  • Sender: owner-wri-mathgroup at

To beginwith, Solve has the nasty habbit of returning a nested list of
rules (since it is designed to work for multiple solution cases); use
Flatten to get rid of that:
result = Flatten@
    Solve[{a == 1/(1/r2 + 1/50), 50 == 1/(1/(a + r1) + 1/r2)}, r1, a]

Now result is something like {r1->...function of r2...} which is  a
list with one rule. (If you are serious about Mathematica, go now to
the Mathematica Book and start reading about rules by the way..)

r1/.result returns the expression so simply do 
That's it!


"1.156" <rob at> wrote in message news:<b1f6h6$ara$1 at>...
> I'd sure like to find out how to clean up a process I do a lot.  Namely, get
> a solution to some set of equations and then plot the result. For example,
> I recently did
> result = Solve[{a == 1/(1/r2 + 1/50), 50 == 1/(1/(a + r1) + 1/r2)}, r1, a]
> This gives {{r1 -> (a function of r2)  }}
> Then, I plot it by
> Plot[ (this function of r2), {r2, startvalue, stopvalue}]
> where I carefully type in this function.  I feel sure you Mathematica pros
> don't have to do that so I have made several feeble attempts to automate
> this over the years. They fail because I still don't have a clue how Mathematica works.
> Here's my last attempt:
> Plot[ result /. %]
> Mathematica just hisses and prints out tons of error messages, none of which mean
> a thing to me.
> Surely there is a way to get this plot without having to type the Solve[] result
> into Plot[].  Any hints would be appreciated, as usual.
> Rob

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