How do you use the half angle formula for
sin (5π/12)?

The half angle formula is:
sin x/2 = +/- √ (( 1-cos x ) / 2 )
5π/12 = 5π/6 : 2
sin 5π/12 = +/- √(( 1 - cos ( 5π/6)) / 2 ) 
cos 5π/6 = cos 150° = -√3/2

≈ 0.966 ( this is sin 5π/12 )

(-8,5) bu tun not sure ok