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From: Eduardo M. A. M.Mendes on 8 Aug 2010 07:19
Hello

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.

Many thanks

Ed

PS. I am new to Mathematica.

From: Bill Rowe on 9 Aug 2010 05:13
On 8/8/10 at 7:20 AM, emammendes(a)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.


From: Alexei Boulbitch on 9 Aug 2010 05:15
Hi, Ed, try this:

Plot[Solve[{5*x + 4*y == 12}, y][[1, 1, 2]], {x, 0, 2}]

Have fun, Alexei

Hello

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.

Many thanks

Ed

PS. I am new to Mathematica.



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From: Bob Hanlon on 9 Aug 2010 05:15

This works; however, the Solve is evaluated repeatedly

n = 0; Plot[y /.
Solve[{n++; 5*x + 4*y == 12}, y],
{x, 0, 2}]

n

78

Adding Evaluate results in a single evaluation of the Solve

n = 0; Plot[Evaluate[y /.
Solve[{n++; 5*x + 4*y == 12}, y]],
{x, 0, 2}]

n

1

So use

Plot[Evaluate[y /.
Solve[5*x + 4*y == 12, y]],
{x, 0, 2}]


Bob Hanlon

---- "Eduardo M. A. M.Mendes" <emammendes(a)gmail.com> wrote:

=============
Hello

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.

Many thanks

Ed

PS. I am new to Mathematica.



From: Tomas Garza on 9 Aug 2010 05:15
Bear in mind that Solve will yield a list of all the solutions in terms of =
rules of the form {y -> x}. So you want to choose first which of the soluti=
ons you want to plot. You might try something like

In[1]:== sol==Solve[{5*x+4*y====12},{y}]
Out[1]== {{y->1/4 (12-5 x)}}

In[2]:== sol[[1]]
Out[2]== {y->1/4 (12-5 x)}

In[3]:== Plot[y/.sol[[1]],{x,0,2}]


> Date: Sun, 8 Aug 2010 07:20:10 -0400
> From: emammendes(a)gmail.com
> Subject: How to use the result of Solve in Plot?
> To: mathgroup(a)smc.vnet.net
>
> Hello
>
> 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.
>
> Many thanks
>
> Ed
>
> PS. I am new to Mathematica.
>

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