The known vector a = (COS α, sin α) B = (COS β, sin β), |a-b | = 5 / 5 under 2 roots Find the value of COS (α - β)

The known vector a = (COS α, sin α) B = (COS β, sin β), |a-b | = 5 / 5 under 2 roots Find the value of COS (α - β)

|A-B | = 5 / 5 under 2 roots
|a-b|^2=4/5
|(cosa-cosb,sina-sinb)|^2=4/5
(cosa-cosb)^2+(sina-sinb)^2=4/5
cos(a-b) =3/5