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Re: How do I assign the solution obtained by FindRoot to a variable?

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  • Subject: [mg128480] Re: How do I assign the solution obtained by FindRoot to a variable?
  • From: Murray Eisenberg <murray at math.umass.edu>
  • Date: Wed, 24 Oct 2012 03:31:07 -0400 (EDT)
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On Oct 23, 2012, at 12:54 AM, NS Lin <nansheng.lin at gmail.com> wrote:

> I would like to store the root/solution obtained by the FindRoot[] function to a variable.  How can I do that?
>


This is a faq, but usually Mathematica novices ask it about Solve. The situation is similar.

   soln = x /. FindRoot[Cos[x] - x, {x, 0.5}]

That works because FindRoot returns the one-element list {x -> 0.739085}.

In the case of Solve, you may obtain more than one solution, e.g.:

  Solve[x^2 - x == 0, x]
{{x -> 0}, {x -> 1}}

In that situation, you'll need to pick out which solution you want, e.g.:

  x /. Solve[x^3 - x == 0, x][[2]]

Notice that the result of Solve is a list of one-element lists. This is so even when the equation has only a single solution, e.g.:

  Solve[3 x == 1, x]
{{x -> 1/3}}

In that case you can either use indexing to form Solve[3 x == 1, x][[1]] before using "x /. ", or just use First:

  x /. First@Solve[3 x == 1, x]

The situation is quite similar for DSolve.

---
Murray Eisenberg                           murray at math.umass.edu
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