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I'm asking Mathematica to compute:

Integrate[
 Module[{r = {-x, -y, z}}, r/Norm[r]^3],
 {x, -a/2, a/2},
 Assumptions -> Elements[y, Reals] && z > 0 && a > 0]

But the result seems to completely ignore my assumptions (I also tried Assuming):

results

What am I doing wrong?

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    $\begingroup$ It's Element, not Elements. You can tell you've mistyped the command because it's color coded blue. $\endgroup$
    – bill s
    Jul 4, 2019 at 14:30
  • $\begingroup$ I knew it was something incredibly obvious! Thanks a lot for help. $\endgroup$
    – mikea
    Jul 4, 2019 at 14:50

1 Answer 1

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Try 

Module[{r = {-x, -y, z}},Integrate[r/Norm[r]^3, {x, -a/2, a/2},Assumptions -> Element [y, Reals] && z > 0 && a > 0]]
(*{0, -((2 a y)/((y^2 + z^2) Sqrt[a^2 + 4 y^2 + 4 z^2])), (2 a z)/((y^2 + z^2) Sqrt[a^2 + 4 y^2 + 4 z^2])}*)
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