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MathGroup Archive 1998

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Re: calculation



Saeed Esmaily Rashid wrote:
> 
> Hello!
> 
> My name is Saeed and i'm studying physics. I have a problem which ihope
> someone can help me with it. 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. the question is that
> is there any way to optimize this equation or using another function 


Hello, 
      This is a numerical problem so use FindRoot, not Solve, 

FindRoot[.001422409738*R^(73/80)*(R^(1/2)-1)^(-13/40) == .04,{R,2}]

and you'll get 

{R->81.268}

Plotting the function also helps.  Try

Plot[.001422409738*R^(73/80)*(R^(1/2)-1)^(-13/40) - .04,{R,0,100}]

and you'll find another root close to the singularity

FindRoot[.001422409738*R^(73/80)*(R^(1/2)-1)^(-13/40) ==
.04,{R,1.00001}]

{R->1.00007}

You can also try using the option WorkingPrecision (if you know  that it
is justified).

John Jowett
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