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Re: Simplify stuck in simple expression

• To: mathgroup at smc.vnet.net
• Subject: [mg50143] Re: [mg50127] Simplify stuck in simple expression
• From: Bob Hanlon <hanlonr at cox.net>
• Date: Tue, 17 Aug 2004 05:01:24 -0400 (EDT)
• Reply-to: hanlonr at cox.net
• Sender: owner-wri-mathgroup at wolfram.com

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

> 
> From: carlos at colorado.edu (Carlos Felippa)
To: mathgroup at smc.vnet.net
> Date: 2004/08/16 Mon AM 06:45:34 EDT
> To: mathgroup at smc.vnet.net
> Subject: [mg50143] [mg50127] Simplify stuck in simple expression
> 
> Hi - I wonder why
> 
> Simplify[(2 - Sqrt[72*(1+a*b)^2+4])/(2*a)];
> 
> in v. 4.2 gets stuck. It does not reduce the expression to 
> 
> (1 - Sqrt[18*(1+a*b)^2+1])/a;
> 
> FullSimplify and declaring-variable-attribute variations do not 
> work.   What is necessary for the simplification to happen?
> 
>