       Re: NEWBIE: How do I use the results of a Solve?

• To: mathgroup at smc.vnet.net
• Subject: [mg95994] Re: NEWBIE: How do I use the results of a Solve?
• From: Jean-Marc Gulliet <jeanmarc.gulliet at gmail.com>
• Date: Sat, 31 Jan 2009 06:44:35 -0500 (EST)
• Organization: The Open University, Milton Keynes, UK
• References: <gm0q3d\$rm2\$1@smc.vnet.net>

```In article <gm0q3d\$rm2\$1 at smc.vnet.net>, Tim Heger <timheger at yahoo.com>
wrote:

> I can't figure out how to use the results of Solve I just created  as a new
> variable.
>
> Here's a simple example of what I am trying to do:
>
> Solve[some equation, x]
>
> Results in {x->12}
>
> I then want to use the x in  y =  x * 5 for example....
>
> Obviously when I try to use this it fails because Mathematica sees the
> equation as y = {x->12} * 5  and not y = 12 *5 ...
>
> How do I change the results from the Solve from a rule to an actual value I
> can then use?

The function Solve returns a list of transformation rules (possibly an
empty list or just one) that can be further applied to other expressions
via the replacement operator /. as you can see in the following examples:

In:= sol = Solve[x - 12 == 0, x]

Out= {{x -> 12}}

In:= y = x*5 /. sol[]

Out= 60

In:= sol = Solve[x^2 - 12 == 0, x]

Out= {{x -> -2 Sqrt}, {x -> 2 Sqrt}}

In:= y = x*5 /. sol

Out= {-10 Sqrt, 10 Sqrt}

The tutorial "Solving Equations" might be worth reading. It is available
on the web at

http://reference.wolfram.com/mathematica/tutorial/SolvingEquations.html

or directly from within the documentation center at

tutorial/SolvingEquations

Regards,
--Jean-Marc

```

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