       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)
• 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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