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Creating a function that is a derivative of another function??

  • To: mathgroup at smc.vnet.net
  • Subject: [mg86718] Creating a function that is a derivative of another function??
  • From: Eric <eric.r.george at aero.org>
  • Date: Wed, 19 Mar 2008 05:25:38 -0500 (EST)

I have a function:

W[t_] := a*t + b*t^2 + c*t^3

I want the derivative of this funtion wrt t as another function.  The following seems like it works:

dW[t_] := D[W[t], t]

dW[t]
a + 2 b t + 3 c t^2

But something isn't right??

dW[0]
General::ivar: 0 is not a valid variable. >>
(output line has a partial derivative symbol, subscript 0 followed by a 0)

The description of the General::ivar error isn't help either??

Any idea what's going on here??  How can I assign the derivative of a function as another funtion?!

Thanks
Eric


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