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MathGroup Archive 2004

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Oddities of Plot[]

  • To: mathgroup at smc.vnet.net
  • Subject: [mg51123] Oddities of Plot[]
  • From: Roger Mason <rmason at esd.mun.ca>
  • Date: Tue, 5 Oct 2004 04:37:12 -0400 (EDT)
  • Organization: Memorial University of Newfoundland
  • Sender: owner-wri-mathgroup at wolfram.com

Hello,

Playing around in Mathematica 4 I came across this oddity:

In[103]:=
circle = Solve[r^2 == (x - h)^2 + (y - k)^2, y]
Out[103]=
{{y -> k - Sqrt[-h^2 + r^2 + 2 h x - x^2]}, {y -> 
        k + Sqrt[-h^2 + r^2 + 2 h x - x^2]}}

In[104]:=
primitivelow = y /. circle[[1]] /. {r -> 1 , h -> 0, k -> 0};
primitivehigh = y /. circle[[2]] /. {r -> 1 , h -> 0, k -> 0};
primitive = {primitivehigh, primitivelow}
Out[106]=
{1 - x^2, -1 - x^2}

I don't see why this fails

In[107]:= 
Plot[primitive, {x, 0, 1}]
Plot::"plnr": "primitive is not a
machine-size real number at x = 4.166666666666666`*^-8."

while this works

In[108]:=
Plot[{1 - x^2, -1 - x^2}, {x, 0, 1}]

Thanks,

Roger Mason


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