If you want  then since , then 

But I think you want .  You need to put parentheses
so we can tell what you want.

We use the half-angle formula:



Now 

So 



Now we can determine the sign by realizing that since  is in
quadrant I, that half of it, , will also be in the first quadrant,
and in quadrant I, and all trig functions of angles in quadrant I are positive.

So we have



Edwin