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

• To: mathgroup at smc.vnet.net
• Subject: [mg39327] Re: [mg39303] Simplify[Abs[x],x<0]]
• From: Tomas Garza <tgarza01 at prodigy.net.mx>
• Date: Tue, 11 Feb 2003 04:42:34 -0500 (EST)
• References: <200302100607.BAA23841@smc.vnet.net>
• Sender: owner-wri-mathgroup at wolfram.com

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.

In[1]:=
Simplify[Abs[-x], x > 0]
Out[1]=
x

Or else, use

In[2]:=
Simplify[ComplexExpand[Abs[x]], x < 0]
Out[47]=
-x

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

Tomas Garza
Mexico City
----- Original Message -----
From: "Uri Zwick" <zwick at cs.tau.ac.il>
To: mathgroup at smc.vnet.net
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
>
>
>

• References:
  • Simplify[Abs[x],x<0]]
    • From: Uri Zwick <zwick@cs.tau.ac.il>