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Re: How to use the result of Solve in Plot?

  • To: mathgroup at smc.vnet.net
  • Subject: [mg111653] Re: How to use the result of Solve in Plot?
  • From: Bill Rowe <readnews at sbcglobal.net>
  • Date: Mon, 9 Aug 2010 05:13:07 -0400 (EDT)

On 8/8/10 at 7:20 AM, emammendes at gmail.com (Eduardo M. A. M.Mendes)
wrote:

>I want to use the result of Solve[{5*x+4*y==12},{y}] in
>Plot[.,{x,0,2}]. Plot[Solve[.],{x,0,2}] does not work.

Solve returns results as a list of replacement rules. So, what
you need to do is:

sol = Solve[{5*x + 4*y == 12}, {y}];
Plot[y /. First[sol], {x, 0, 5}]

Note, the use of First. Since Solve is intended to solve for
more than one variable and provide all possible solutions the
result is a nested list rather than a flat list. In this
particular case where there is only one variable and only one solution

Plot[y/.sol, {x, 0, 5}]

will create the same plot. But in general, you will need to
select one of the solutions using something like First. So, it
is a good idea to not get into the habit of thinking Plot[y/.sol
... will always work.



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