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Re: Failure to Evaluate?

• To: mathgroup at smc.vnet.net
• Subject: [mg49859] Re: Failure to Evaluate?
• From: "Zhu Chongkai" <zhuchongkai at online.sh.cn>
• Date: Wed, 4 Aug 2004 10:46:39 -0400 (EDT)
• References: <cen8is$nt$1@smc.vnet.net>
• Sender: owner-wri-mathgroup at wolfram.com

What is the "D" in the defination of h?

D[f, x] gives the partial derivative ?f/?x, but D[f]/D[g] means nothing.


"Scott Guthery" <sguthery at mobile-mind.com>

> 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?
>
> h/.#1->1//N sets #1 to 1 and gives
>
> (1 1 + 1 &)/(1 + 7 &)
>
> but still doesn't evaluate to 1/4.
>
> What gives? What's the Right Stuff?
>
> Thanks for any insight.
>
> Cheers, Scott
>