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/

```

• Prev by Date: Re: how do I display "Power[z,-1]" in an exponetial form?
• Next by Date: Re: Newbie Plot Question
• Previous by thread: Forcing a Derivative
• Next by thread: Re: Re: Forcing a Derivative