A sequence of positive real numbers a_1, a_2, a_3... has the property that for i>=2, each a_i is equal to the sum of all the previous terms. If a_19=99, then what is a_20?

Thanks in advance.

99 = F + 18D..............................(1) 19/2 * [2F + 18D] = 99................(2), solve for F, D

F = - 1683 / 19 - This is the first term D = 198 / 19 - This is the common difference

20th term = - 1683 / 19 + [(198 / 19)*19] 20th term =2079 / 19