The 3rd, 4th, and 5th terms of a geometric progression are t+1, t+8 and t+18 respectively.determine common difference, first term, sum of the first 12 terms?
(t+8)^2 = (t+1)(t+18)
t = 46/3
so the three terms are
49/3, 70/3, 100/3
r = 10/7
so since ar^2 = 49/3, a = 7^4/300
S12 = a(r^12 - 1)/(r-1) = (7^4/300)((10/7)^12 - 1)/(10/7 - 1)
Somehow I think you mangled this question