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