a.b
=(cos(3x/2),sin(3x/2)).(cos(x/2),-sin(x/2))
=cos(3x/2).cos(x/2)- sin(3x/2).sin(x/2)
=cos2x
|a+b|^2
=(cos(3x/2)+cos(x/2))^2+ (sin(3x/2)-sin(x/2))^2
= 2 + 2(cos(3x/2)cos(x/2)- sin(3x/2)sin(x/2) )
=2 + 2cos2x
|a+b| =√(2 + 2cos2x)