Hi, I am tutoring the UK curriculum for AS (11th grade) Level Mathematics and came upon this question which has stumped me. Any help would be appreciated.

An infinite geometric series has a first term a and sum to infinity b, where b ≠ 0. Prove that a lies between 0 and 2b.

Now I can work out that the StI is b = a/(1 - r) but I’m not sure how to proceed from here. Any help would be most appreciated. Thanks.