The 2nd and 5th terms of a G.p are -7_and 56 find the common ratio, the first term, the sum of the first terms?
r^3 = 56/(-7) = -8
r = -2
so a = -7/r = 3/2
and the sum of the first n terms is, as usual,
Sn = a(r^n - 1)/(r-1)
r = -2
so a = -7/r = 3/2
and the sum of the first n terms is, as usual,
Sn = a(r^n - 1)/(r-1)