half angle + or negative?!

[kpx001]

hey for the problem
sin = 3/5 0< <pi/2

sin theta/2 = +/- root(1-(4/5) / 2 ) = root (1 / 10 )
and
cos theta/2 = +/- root(1 + (4/5) /2 ) = root (9/10)

how do i know if its + or - ? i know it has to do with the quadrant it lies on but im clueless

 

[tkhunny]

For angles in [0,pi/2], half that angle is in [0,pi/4]. That's still Quadrant I.