If `theta` is the angle between the unit vectors `vec a` and `vec b`, then prove that `sin (theta)/(2) = 1/2|vec a - vec b|`

Explanation

i. `(veca+vecb).(veca+vecb)= |veca|^(2)+|vecb|^(2)+2veca.vecb`
`=1 +1+2(1)(1) cos theta`
`cos(theta/2)=1/2|veca+vecb|`
ii. `(veca-vecb).(veca-vecb)=|veca|^(2)=|vecb|^(2)-2veca.vecb`
`= 1+1-2(1)(1)cos theta`
`=2-2costheta`
`or sin"theta/2=1/2|veca+vecb|`