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MathGroup Archive 2003

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Re: Simplify[Abs[x],x<0]]

  • To: mathgroup at
  • Subject: [mg39327] Re: [mg39303] Simplify[Abs[x],x<0]]
  • From: Tomas Garza <tgarza01 at>
  • Date: Tue, 11 Feb 2003 04:42:34 -0500 (EST)
  • References: <>
  • Sender: owner-wri-mathgroup at

I do not wish to appear flippant, but if you wish to calculate Abs[x] and
you know that x < 0, then evaluate Abs[-x] with the assumption that x >0:
Abs[x] given that x <0 is equal to Abs[-x] given x > 0.

Simplify[Abs[-x], x > 0]

Or else, use

Simplify[ComplexExpand[Abs[x]], x < 0]

to make sure you are talking of real x (I presume).

Tomas Garza
Mexico City
----- Original Message -----
From: "Uri Zwick" <zwick at>
To: mathgroup at
Subject: [mg39327] [mg39303] Simplify[Abs[x],x<0]]

> Hi,
> Simplify[ Abs[x] , x>0 ] returns x.
> But, Simplify[ Abs[x] , x<0] returns Abs[x], and not -x.
> Why is that?
> Uri

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