Consuder the sequence that begins 40, 20, 10, 5

The explicit formula for a geometric sequence is usually written as: an = ar (n - 1) where a=a1 and r=common ratio a=80r = (1/2) an = 80 (1/2) (n=1) 80, 40, 20, 10, 5, 2 1/2, 1 1/4, 5/8, 5/16, ... note: 0.625 = 5/8 For some applications, the first element is A0, so you may encounter that preference.

The next on the sequence is 2.5