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MathGroup Archive 2004

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Re: Forcing a Derivative

  • To: mathgroup at smc.vnet.net
  • Subject: [mg50765] Re: [mg50753] Forcing a Derivative
  • From: Andrzej Kozlowski <andrzej at akikoz.net>
  • Date: Sun, 19 Sep 2004 21:39:21 -0400 (EDT)
  • References: <200409190756.DAA17973@smc.vnet.net>
  • Sender: owner-wri-mathgroup at wolfram.com

On 19 Sep 2004, at 16:56, Scott Guthery wrote:

> *This message was transferred with a trial version of CommuniGate(tm) 
> Pro*
> How does one force Derivative[n] to actually take the derivative?
>
> For example if ...
>
> f[x_] = x^2 + 7
>
> g[x_]=3x^3 + 23
>
> then
>
> Derivative[2][f * g]
>
> just puts a couple of primes on the product rather than actually 
> computing the dervative.
>
> Thanks for any insight.
>
> Cheers, Scott
>
>
Mathematica does not understand that you mean by f*g the function that 
takes x to f(x)*g(x). There are various ways to deal with this issue, 
but the simplest are:

Derivative[2][f[#]*g[#] &][x] // Expand

60*x^3 + 126*x + 46

or


Expand[D[f[x]*g[x], {x, 2}]]

60*x^3 + 126*x + 46



Andrzej Kozlowski
Chiba, Japan
http://www.akikoz.net/~andrzej/
http://www.mimuw.edu.pl/~akoz/


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