# Assign a Derivate to a new fuction variable

I'm having a new problem. I'm trying to create a new function from a Derivate of another function, but i'm can't getting it. Someone could help me? I'm thinking that is something in the syntax of the progam that i'm not doing in the right way...Thanks in advance guys]1

• It appears from the error that velt is already defined. Please clear all definitions (or quit kernel) and define it again. There should be no syntax issue here because defining a function as a derivative of another function is quite legitimate. For instance fun[t_] = D[Sin[t] Exp[t], t] works as expected. – Lotus May 30 at 17:21
• You need something like velt[x_] := Module[{t}, D[newpostt[t], t] /. t -> x]; because to need to differentiate w.r.t. a symbolic variable and not a number. – Somos May 30 at 23:28