Note to closers : This is a question about a Programming language (Mathematica), and not about a discipline/science (mathematics).
Why is
N[D[Sin[x], x] /. x -> Pi/4]
(*
Out -> 0.707107
*)
but
N[D[Abs[x], x] /. x -> Pi/4]
(*
Out -> Derivative[1][Abs][0.785398]
*)
?
And what is the better way to force a numerical result?