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[1]:= sol = Solve[x - 12 == 0, x] Out[1]= {{x -> 12}} In[2]:= y = x*5 /. sol[[1]] Out[2]= 60 In[3]:= sol = Solve[x^2 - 12 == 0, x] Out[3]= {{x -> -2 Sqrt[3]}, {x -> 2 Sqrt[3]}} In[4]:= y = x*5 /. sol Out[4]= {-10 Sqrt[3], 10 Sqrt[3]} 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