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Re: How to make the same two expressions

  • To: mathgroup at smc.vnet.net
  • Subject: [mg119585] Re: How to make the same two expressions
  • From: Peter Breitfeld <phbrf at t-online.de>
  • Date: Sat, 11 Jun 2011 03:58:41 -0400 (EDT)
  • References: <isss9i$jg1$1@smc.vnet.net>

Mariano Pierantozzi wrote:

> Hi everyone,
> i've this problem with Mathemtica:
> i've got 2 expression depending on a and b:
> expr1= -1.0019 - 1. Log[0.00190438 - 0.0249519 b] + (0.000040093 a
> Log[0.152644/(0.152644 + 2 b)])/b
> expr2=-1.00004 - 1. Log[0.0000378969 - 0.0249519 b] + (0.000040093 a
> Log[0.0030376/(0.0030376 + 2 b)])/b
> I want to make the same this two expressions.
> The problem is that if i try to solve this is one equation in two parameter:
> a and b.
> So i think to put inside some value of a and b so i make equal the two
> expression less than 10^-12 (for example)
> I've to do a list of value:
> b can vary beetwen -0.0015188 < b < 0.0015188 (the domain of logarithmic
> function) and a must vary for example  -2000 to 2000.
> I can not find a waydo it. I try with list, and put the value inside, but
> i've some problem.
> if anyone can help I would be grateful...
> Thank you!
>
> Mariano Pierantozzi
> PhD student

You may use FindInstance:

lsg=FindInstande[expr1==expr2, {a,b}, Reals]
Out= {{a -> 76.1255, b -> -0.00105906}}

expr1-expr2/.lsg
Out= {-8.88178*10^-16}

-0.0015188 < b < 0.0015188 /. lsg
Out= {True}

-- 
_________________________________________________________________
Peter Breitfeld, Bad Saulgau, Germany -- http://www.pBreitfeld.de


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