Let a = [1, 1, 0], b = [3, 2, 1], and c = [1, 0, 2]. Find the angle between:

cos (α - β) = cosα cos β + sin α sin β. Obtain this by using a = [cos α, sin α], b = [cos β, sin β] where 0 ≤ α ≤ β ≤ 2π.