Half Angle Trigonometric Identities Formula
sin (X/2) = + or - SQRT [ (1 - cosX) / 2 ]
cos (X/2) = + or - SQRT [ (1 + cosX) / 2 ]
tan (X/2) = + or - SQRT [ (1 - cosX) / (1 + cosX) ] = sinX / (1 + cosX) = (1 - cosX) / sinX
Trig half angle identities calculation for sine and cosine angle from the derived forumla is made easier here.
For example, When angle = 60o, sin( 60/2 ) =0.5, cos( 60/2 ) =0.86603, tan( 60/2 ) =0.57735.