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Re: Integrate is driving me crazy, please help!
*To*: mathgroup at smc.vnet.net
*Subject*: [mg56216] Re: Integrate is driving me crazy, please help!
*From*: Paul Abbott <paul at physics.uwa.edu.au>
*Date*: Tue, 19 Apr 2005 04:55:35 -0400 (EDT)
*Organization*: The University of Western Australia
*References*: <d3vn1h$jm9$1@smc.vnet.net>
*Sender*: owner-wri-mathgroup at wolfram.com
In article <d3vn1h$jm9$1 at smc.vnet.net>,
Jim Martin <jim.martin at utah.edu> wrote:
> Hello Mathematica Experts:
>
> I am a biomechanist and work mostly in the area of muscle contraction. I
> do a lot of numerical computations using excel, but right now I need an
> analytical solution that represents force as a function of position
> integrated over a shortening amplitude. I downloaded a trial version of
> Mathematica and have tried to obtain a solution for this:
>
> Integrate[(hillb*((f0 + hilla)/(2*pi*f*a*Cos(ArcSin(x/a)) + hillb))) -
> hilla, {x, -a, a}]
Instead of 2*pi*f*a*Cos(ArcSin(x/a)) you mean
2 Pi f a Cos[ArcSin[x/a]] + hillb
(In StandardForm, pi needs to be capitalised and square brackets are
required).
> Mathematica returns this:
> (-4 a ArcSin Cos f hilla pi + (f0 + hilla) hillb (-Log[hillb - 2 a
> ArcSin Cos f pi] + Log[hillb + 2 a ArcSin Cos f pi]))/(4 a ArcSin
> Cos f pi)
In this output you will see expressions like
ArcSin Cos f pi
which make no sense. Correcting the integrand, and massaging the result
from Integrate, I get
((f0 + hilla) hillb (Pi (Sqrt[hillb^2 - 4 a^2 f^2 Pi^2] - hillb) +
2 hillb ArcTan[(2 a f Pi)/Sqrt[hillb^2 - 4 a^2 f^2 Pi^2]]))/
(2 f Pi Sqrt[hillb^2 - 4 a^2 f^2 Pi^2]) - 2 a hilla
> In a sample data set, hilla=3, hillb=50, f0=8, a=1, f=1
For this dataset, numerical evaluation gives 14.0372.
> I can numerically integrate this function and obtain a value for that
> sample data set of 14.04.
But how did you do this and get this correct result if you used the
wrong integrand?
> In Mathematica, Log is Log to base e, right (LN in excel)? Did I use
> variables that have intrinsic functions in Mathematica? Maybe I am
> misunderstanding the output with regard to implicit parentheses etc.
Wrong type of parentheses. If you use TraditionalForm for input and
output (Cell | Default Input FormatType, etc.) you _can_ enter
expressions using () instead of [].
Cheers,
Paul
--
Paul Abbott Phone: +61 8 6488 2734
School of Physics, M013 Fax: +61 8 6488 1014
The University of Western Australia (CRICOS Provider No 00126G)
35 Stirling Highway
Crawley WA 6009 mailto:paul at physics.uwa.edu.au
AUSTRALIA http://physics.uwa.edu.au/~paul
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