If the nonzero vectors a and B satisfy: | B | = 1, and the angle between B and B-A is 60 degrees, the value range of | a |?

If the nonzero vectors a and B satisfy: | B | = 1, and the angle between B and B-A is 60 degrees, the value range of | a |?

b(b-a)/(|b|×|b-a|)=1/2
Simplification
|a|²-2|a|×cosA+1=0
Namely
|a|=cosA