text: a question on geometric progression..? if (a+b)/(a-b)=(b+c)/(b-c), show that a, b and c form a geometric sequence. (a+b)/(a-b)=(b+c)/(b-c) (a+b)(b-c) = (b+c)(a-b) ab-ac+b2-bc = ab-b2+ac-bc b2 + b2 = ac + ac 2b2 = 2ac b2 = ac bb = ac b/a = c/b  if a, b and c are in a geometric sequence, the ratio of the numbers would be the same.