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MathGroup Archive 1998

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Re: calculation


  • To: mathgroup@smc.vnet.net
  • Subject: [mg11997] Re: calculation
  • From: bawolk@ucdavis.edu (Bruce Wolk)
  • Date: Fri, 17 Apr 1998 03:40:33 -0400
  • Organization: University of California, Davis
  • References: <6gr69a$8lo@smc.vnet.net>

On 12 Apr 1998 15:56:26 -0400, Saeed Esmaily Rashid
<saeedr@stud.ntnu.no> 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 in
>Mathematica 3.0 to make it faster to calculate?
>
>Regards 
>
>
Solve is often not successful since it tries to give explicit
algebraic solutions to equations.  FindRoot is a better alternative
for your equation.  You have to guess at a starting point, but here it
is obvious that the answer is close to 1.  FindRoot is very fast,
although if your guess is too far off it too can fail.

In[58]:=
sol=FindRoot[.001422409738*R^(73/80)*(R^(1/2)-1)^(-13/40)==.04,{R,
1.001}]
Out[58]=
{R->1.00007}
In[59]:=
N[R/.sol,10]
Out[59]=
1.000069591

Hope this is helpful.

Cheers,

Bruce



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