# Behaviour of Simplify when used with ArcTan and ComplexExpand along with assumptions

I am trying to simplify the slope of vectors with symbolic coordinates {x,y} using ArcTan[x,y] I am working with real X and Y

I tried the following :

ClearAll@L;
ArcTan[L, L]
ComplexExpand@ArcTan[L, L]
Assuming[Element[L, Reals] && L > 0, ComplexExpand@ArcTan[L, L]]
% // Simplify
Simplify[ComplexExpand@ArcTan[L, L], {Element[L, Reals], L > 0}]


and got the following output:

ArcTan[L, L]
Arg[(1 + I) L]
Arg[(1 + I) L]
Arg[(1 + I) L]
\[Pi]/4


Why is there a difference between the last line

Simplify[ComplexExpand@ArcTan[L, L], {Element[L, Reals], L > 0}]


and the third-last line?

Assuming[Element[L, Reals] && L > 0, ComplexExpand@ArcTan[L, L]]

-
The documentation states that "Assuming affects the default assumptions for all functions that have an Assumptions option." Since ComplexExpand does not have an Assumptions option, it does not use the assumptions you specified in Assuming –  Simon Woods Feb 20 '14 at 20:04
Thanks you were right. On removing the assumption L>0 from the Simplify[] and the results match! –  my account_ram Feb 22 '14 at 13:57
@SimonWoods - Can you add it as an answer so that I can Accept it –  my account_ram Feb 22 '14 at 13:57

Since ComplexExpand does not have an Assumptions option, it does not use the assumptions you specified in Assuming.