i ran into a bug in my codes

my alpha is good for angle 90 to -90 degree.

but when my velocity vector goes to 91 to 180 degree, my alpha is storing the acute angle made with negative x axis.

for example angle made = 150 degree, but my alpha shows -30 degree in the field.

is it because of

[s]1. the conversion from degree to radian ?[/s]
2. the Math.sin Math.cos Math.tan formula ? is there another formula for the angle always made with positive x axis ?

attached is my xml :)