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# 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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