# Assumptions with D

I want to give assumptions for the D function. Say I want to calculate $\frac{d|x|}{dx}$ for $x>0$, which is 1.

I write D[Abs[x], x, Assumptions -> x > 0] which gives

D::optx: Unknown option Assumptions in D[Abs[x],x,Assumptions->x>0]. >> Why doesn't D take assumptions?

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Because Assumptions is not a valid option for D[]; see Options[D]. What you want is FullSimplify[D[Abs[x], x], x > 0]. –  Ｊ. Ｍ. Jul 7 '12 at 4:25