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Re: Calculating derivate in Mathematica 9 giving wrong result

  • To: mathgroup at smc.vnet.net
  • Subject: [mg131428] Re: Calculating derivate in Mathematica 9 giving wrong result
  • From: Bill Rowe <readnews at sbcglobal.net>
  • Date: Fri, 5 Jul 2013 06:01:26 -0400 (EDT)
  • Delivered-to: l-mathgroup@mail-archive0.wolfram.com
  • Delivered-to: l-mathgroup@wolfram.com
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On 7/3/13 at 5:01 AM, jhlawley at gmail.com (James) wrote:

>In[5]:= D[(x^2 - 2)/(x^3 + 6), x]

>Out[5]= -((3 x^2 (-2 + x^2))/(6 + x^3)^2) + (2 x)/(6 + x^3)

>That is from Mathematica 9.

>The wolfram alpha site and my textbook give this answer:

>d/dx((x^2+x-2)/(x^3+6)) = (-x^4-2 x^3+6 x^2+12x+6)/(x^3+6)^2

>Please tell me what I am doing wrong.

You asked Mathematica to solve a different problem than what you
asked WolframAlpha to solve

Notice in the numerator of your In[5] above is x^2 - 2 while the
numerator of what you sent to WolframAlpha is x^2 + x - 2

That is:

In[2]:= D[((x^2 + x - 2)/(x^3 + 6)), x] // Together

Out[2]= (-x^4 - 2*x^3 + 6*x^2 + 12*x + 6)/(x^3 + 6)^2




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