If each term of a geometric progression a_{1},a_{2},a_{3}, \ with a_{1} = \frac{1}{8} and a_{2} eq a_{1}, is the arithmetic mean of the next two terms and S_{n} = a_{1} + a_{2} + + a_{n}, then S_{20} - S_{18} is equal to\: Sequence & Series (Mathematics) | DivineJEE