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MathGroup Archive 2006

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Re: How to plot this

  • To: mathgroup at smc.vnet.net
  • Subject: [mg64454] Re: How to plot this
  • From: Jon Harrop <usenet at jdh30.plus.com>
  • Date: Fri, 17 Feb 2006 04:12:35 -0500 (EST)
  • References: <dt1coe$5j$1@smc.vnet.net>
  • Sender: owner-wri-mathgroup at wolfram.com

siegmund natter wrote:
> Can anyone tell why this is wrong?
> 
> eqn1 = x + y == 2*x*y;
> eqn1 /.y -> y[x];
> Plot[y[x], {x, 1, 3}]
> 
> How do I do it correctly? (This is just an example; I want to do this
> with a much more difficult equation)
> Thanks a lot.

You can solve for y to get an expression for y[x]:

  In[1]:= eqn1 = x + y == 2*x*y;

  In[2]:= y[x_] = First[y /. Solve[eqn1, {y}]]

             x
  Out[2]= --------
          -1 + 2 x

which you can then plot:

  In[3]:= Plot[y[x], {x, 1, 3}];

You could also use something like InequalityPlot to highlight the same
border:

  In[4]:= << Graphics`InequalityGraphics`

  In[5]:= InequalityPlot[x + y < 2*x*y , {x, -3, 3}, {y, -3, 3}];

-- 
Dr Jon D Harrop, Flying Frog Consultancy Ltd.
http://www.ffconsultancy.com/products/ocaml_for_scientists/chapter1.html


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