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Re: Plot results not right

  • To: mathgroup at smc.vnet.net
  • Subject: [mg89899] Re: Plot results not right
  • From: Jean-Marc Gulliet <jeanmarc.gulliet at gmail.com>
  • Date: Tue, 24 Jun 2008 03:27:15 -0400 (EDT)
  • Organization: The Open University, Milton Keynes, UK
  • References: <g3nhbc$g6$1@smc.vnet.net>

BrenB wrote:

> I'm studying Calculus, and I'm trying to reproduce, in Mathematica,
> the graph results of an equation in my book.
> 
> Plot[2 x - 3 x^(2/3) + 4, {x, -1, 6}]
> 
> The output graph is correct, except there should be graphing in the
> 2nd and 3rd quadrant, and there is none.
> 
> Is there another way to enter this equation into Mathematica to get
> the correct graph results?

Are you sure that the function you want to plot is this one? I might 
have missed something, but, since you mentioned you are taking a class 
in Calculus, I think you are studying real functions of a real variable 
(i.e. functions from R to R).

The function f[x_] := 2 x - 3 x^(2/3) + 4 is defined on R+ only (i.e. 
for x positive or null) and is positive on this interval. Thus the 
function is always above the x-axis and can be drawn (or make sense) 
only for x >= 0. Therefore the graph returned by Plot[] is correct.

Regards,
-- Jean-Marc


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