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:
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> 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/
- Follow-Ups:
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- From: Murray Eisenberg <murray@math.umass.edu>
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- References:
- Forcing a Derivative
- From: "Scott Guthery" <sguthery@mobile-mind.com>
- Forcing a Derivative