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MathGroup Archive 2005

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Re: Plot problem

  • To: mathgroup at smc.vnet.net
  • Subject: [mg61601] Re: Plot problem
  • From: Bill Rowe <readnewsciv at earthlink.net>
  • Date: Sun, 23 Oct 2005 05:46:09 -0400 (EDT)
  • Sender: owner-wri-mathgroup at wolfram.com

On 10/22/05 at 5:11 AM, smilehezuk at gmail.com (smilehezuk) wrote:

>I want to solve these equations and plot. Please tell me how.

>There are two equations;

>Lamda = Sqrt[R A x/B], R = C x (D + E/Lamda) (A,B,C,D,E :
>constants, x : another variable)

>I need to plot R versus x.

First, you need to appreciate the expressions above are not equations in Mathematica. Set ("=") causes the symbol on the lhs to be assigned the value of the evaluated expression on the rhs. Equal in Mathematica is "==" not "="

A second issue is your choice of names for the constants. In general, it is not a good idea to use capital letters for your constants. In particular E is a built in symbol which already has the value 2.7182... The reason for not using capital letters as constants or variables is to avoid conflicts with existing built in symbols or symbols defined in packages you load.

So, your problem could be done as follows:

In[1]:=
lamda = Sqrt[r*a*(x/b)]; 
eq = r == c*x*(d + e/lamda); 
sol = Solve[eq, r];

In your example, lambda can be set to Sqrt[r a (x/b)] since it only appears on the rhs of the second equation.

Now

In[4]:=Length@sol
Out[4]=3

shows there are three solutions to your problem. Picking the first,

In[5]:=f = R /. First[sol]; 

it can be plotted with

Plot[f, {x, minValue, maxValue}]

after defining values for a, b, c, d and e
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