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Re: plot

  • To: mathgroup at smc.vnet.net
  • Subject: [mg110741] Re: plot
  • From: Murray Eisenberg <murray at math.umass.edu>
  • Date: Sun, 4 Jul 2010 03:09:40 -0400 (EDT)

Why only for x > 1? Your quotient (which has a redundant set of 
parentheses around its numerator) evaluates to a negative real when x < 1.

Take, e.g., x = -3:

   Sqrt[x - 1] /. x -> -3 // InputForm
2*I

   (2 x + 1) Sqrt[x + 1] // InputForm
(-5*I)*Sqrt[2]

   (2 x + 1) Sqrt[x + 1)/Sqrt[x - 1] /. x -> -3 // InputForm
-5/Sqrt[2]

And consider:

   (2 x + 1) Sqrt[x + 1]/Sqrt[x - 1] // Simplify // InputForm
(1 + 2*x)/Sqrt[(-1 + x)/(1 + x)]

So if you want the graph to exclude negative x, you'll have to do it 
explicitly.

On 7/2/2010 2:57 AM, agua wrote:
> Hi
> With  Plot[( (2x+1) Sqrt[x+1] ) / Sqrt[x-1] ,{x,-5,5}]
> hoped to obtain a graph only for x>1.
>
> What happened?
> regards.
>

-- 
Murray Eisenberg                     murray at math.umass.edu
Mathematics & Statistics Dept.
Lederle Graduate Research Tower      phone 413 549-1020 (H)
University of Massachusetts                413 545-2859 (W)
710 North Pleasant Street            fax   413 545-1801
Amherst, MA 01003-9305


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