Hi everyone,

I define the function and define new function using the derivative and NIntegrate on it. As an example, I made some simple case.

Clear[f,d]
f[x_?NumericQ, a_?NumericQ] := Sin[a x]
d[x_?NumericQ] := NIntegrate[D[f[x, a], x], {a, 0, 1}]

And try to get the result at some x.

With[{x = 2}, Evaluate@d[x]]

But it does not work...And I got error message

General::ivar: 2 is not a valid variable.

NIntegrate::inumr: The integrand \!(*SubscriptBox[([PartialD]), (2)](f[2, a])) has evaluated to non-numerical values for all sampling points in the region with boundaries {{0,1}}.

How can I fix this?