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MathGroup Archive 2005

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Re: symbolic approximation (formular manipulation)

  • To: mathgroup at smc.vnet.net
  • Subject: [mg55050] Re: [mg54991] symbolic approximation (formular manipulation)
  • From: Bob Hanlon <hanlonr at cox.net>
  • Date: Thu, 10 Mar 2005 05:25:17 -0500 (EST)
  • Reply-to: hanlonr at cox.net
  • Sender: owner-wri-mathgroup at wolfram.com

I haven't tested this much but it should give you a start.

approx[expr_, x_<y_] := 
    Module[{k},
      (Factor[expr/.x->k*y]/.(k+z_)->z)/. 
        k->x/y];

expr=(R1+R2)/(R1(R2+R3));

approx[expr,R1<R2]

R2/(R1*(R2 + R3))

approx[%,R2<R3]

R2/(R1*R3)


Bob Hanlon

> 
> From: Zhe Hu <iamhuzhe at gmail.com>
To: mathgroup at smc.vnet.net
> Date: 2005/03/09 Wed AM 06:34:18 EST
> To: mathgroup at smc.vnet.net
> Subject: [mg55050] [mg54991] symbolic approximation (formular manipulation)
> 
> A formula in electrical engineering can be simplified after assuming,
> e.g. R1<<R2, within 10% error tolerance.
> 
> How this kind of "simplify" or approximation can be performed by 
Mathematica?
> 
> For example, R1<<R2, R2<<R3
> 
> R1+R2                      R2                                      R2
> ---------------  --->  ------------------  ---------> 
------------------
> R1(R2+R3)               R1(R2+R3)                       R1 R3
> 
> 
> 
> I thought about replacing: R1->0.00001*R2, but that would eliminate R1
> completely from the formula (denumerator), which is not favorable in
> some cases.
> 
> 


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