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Re: Failure to Evaluate?

• To: mathgroup at smc.vnet.net
• Subject: [mg49857] Re: Failure to Evaluate?
• From: Bill Rowe <readnewsciv at earthlink.net>
• Date: Wed, 4 Aug 2004 10:46:36 -0400 (EDT)
• Sender: owner-wri-mathgroup at wolfram.com

On 8/3/04 at 1:11 AM, sguthery at mobile-mind.com (Scott Guthery)
wrote:

>f:=Function[#1^2/#1+1]

>g:=Function[#1^4+7]

>h:=D[f]/D[g]

>h[1]

>Gives

>((#1^2/#1)+1 &)/(#1^4+7 &)[1]

>Why doesn't it evaluate by setting #1 to 1 to get 1/4?

Because that would not necessarily be the correct answer.

You defined h as the derivative of f divided by the derivative of g but failed to specify what the derivative was with respect to. That is D takes two arguments, the function to differentiate and a variable, i.e., D[x+2 y, x] != D[x+ 2 y, y]

Since you didn't supply D with sufficient information, it returned the only argument you supplied it unevaluated, which is exactly what you see in the answer.
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