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Re: Re: Simplify stuck in simple expression

• To: mathgroup at smc.vnet.net
• Subject: [mg50171] Re: [mg50164] Re: Simplify stuck in simple expression
• From: Andrzej Kozlowski <akoz at mimuw.edu.pl>
• Date: Wed, 18 Aug 2004 04:34:07 -0400 (EDT)
• References: <cfsi9a$a78$1@smc.vnet.net> <200408180520.BAA06011@smc.vnet.net>
• Sender: owner-wri-mathgroup at wolfram.com

You can use:


FullSimplify[Factor //@ ((2 - Sqrt[72*(1 + a*b)^2 + 4])/
     (2*a))]


(1 - Sqrt[18*a*b*(a*b + 2) + 19])/a


However, as this has the same LeafCount as the expression you desire 
you, would need to find a suitable ComplexityFunction to obtain the 
answer you want.

On 18 Aug 2004, at 07:20, Carlos Felippa wrote:

> Bob Hanlon <hanlonr at cox.net> wrote in message 
> news:<cfsi9a$a78$1 at smc.vnet.net>...
>> expr = (2-Sqrt[72*(1+a*b)^2+4])/(2*a);
>>
>> You can work around the problem by using a temporary change in 
>> variables
>> (x = a*b + 1)
>>
>> Simplify[expr /. b-> (x-1)/a] /. x->a*b+1
>>
>> or
>>
>> Simplify[expr /. a-> (x-1)/b] /. x->a*b+1 // Cancel
>>
>>
>> Bob Hanlon
>>
> Unfortunately it isnt practical.  That expression is inside a deep
> loop that solves thousands of quadratic equations, and the expressions
> under Sqrt keep changing.   I just cooked up a simplified form for
> posting.
>
> Is there an equivalent of FactorTerms that can force a numerical
> factor out of a symbolic square root?
>
>

• References:
  • Re: Simplify stuck in simple expression
    • From: carlos@colorado.edu (Carlos Felippa)