# Re: calculation

• To: mathgroup@smc.vnet.net
• Subject: [mg12014] Re: calculation
• From: Paul Abbott <paul@physics.uwa.edu.au>
• Date: Fri, 17 Apr 1998 03:40:50 -0400
• Organization: University of Western Australia
• References: <6gr69a\$8lo@smc.vnet.net>

```Saeed Esmaily Rashid wrote:

>I have an equation
>
> .001422409738*R^(73/80)*(R^(1/2)-1)^(-13/40) == .04
>
> i'm using the Solve function in Mathematica 3.0 to solve it for R, but
> it calculats endlessly and takes very long time.

I turns out that Mathematica cannot solve the general problem

a*R^p*(R^(1/2)-1)^c == d

It is easy to construct transcendental equations with no simple
closed-form solution.  Perhaps you should look at a plot, e.g.,

In[1]:= eq=.001422409738*R^(73/80)*(R^(1/2)-1)^(-13/40) == .04;

In[2]:= Plot[Evaluate[Subtract@@eq],{R,1,100}];

Then you can use FindRoot to find the numerical solutions you are after:

In[3]:= FindRoot[eq,{R,1.0001}]
Out[3]= {R -> 1.00007}

In[4]:= FindRoot[eq,{R,80}]
Out[4]= {R -> 81.2655}

Cheers,
Paul

____________________________________________________________________
Paul Abbott                                   Phone: +61-8-9380-2734
Department of Physics                           Fax: +61-8-9380-1014
The University of Western Australia            Nedlands WA  6907
mailto:paul@physics.uwa.edu.au  AUSTRALIA
http://www.pd.uwa.edu.au/~paul

God IS a weakly left-handed dice player
____________________________________________________________________

```

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