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

Answer

(i) `cos.(theta)/(2)=(1)/(2)|vec(a)+vec(b)|`
(ii) `sin.(theta)/(2)=(1)/(2)|vec(a)+vec(b)|`