# Working with Conditions or Assumptions in Mathematica with boolean operators

I have the following code:

$Assumptions = {x > 0} b[x_] := x^2 b'[x] > 0  In my (very basic) understanding of Mathematica, this should give me me the Output True, but I get 2 x > 0. I also tried b[x_] := x^2 /; x > 0 and Assuming[x > 0, b'[x] > 0]. I've searched the mathematica help, but without success. What's my basic error and how do I get the desired output? EDIT: The original question is answered, now I wanted to adapt this solution to two variables: c[x_, y_] := x^2 + y$Assumptions = {y > 0}
$Assumptions = {x > 0} Simplify[c[x, y] > 0]  It follows the same logic as the first case, where I now get the desired output, but why not here? I realize that these are probably typical beginners questions, so if you could explain the logic to me or give me a hint where to read up upon this stuff? Neither the Mathematica help nor my university's (very short) guidebook are sufficient for my understanding. - Have you tried Simplify[b'[x] > 0, x > 0] ? – J. M. Apr 27 '11 at 9:43 Simplify works well on the very first result (2x >0). – Phira Apr 27 '11 at 9:46 @J.M. and @user9325 Thanks guys... Can you post this as an answer so I can upvote and accept? – meep.meep Apr 27 '11 at 10:20 $Assumptions = {x > 0} overwrites $Assumptions = {y > 0}. Try $Assumptions = x > 0 && y > 0. – J. M. Apr 27 '11 at 10:46

$Assumptions = {x > 0} b[x_] := x^2 b'[x] > 0  works fine if you apply  Simplify  to the result (2x > 0). Edit: For completeness, I also add the answer of J.M in the comment to the second question. $Assumptions = {x > 0}

  $Assumptions = {y > 0}.  Try $Assumptions = x > 0 && y > 0.