If the 2nd and 5th terms of a geometric progression are -6 and 48 respectively, find the sum of the first four terms.
r^3 = 48/-6 = -8
r = -2
a = -5/r = 3
and as usual
Sn = a(r^n - 1)/(r-1)
r = -2
a = -5/r = 3
and as usual
Sn = a(r^n - 1)/(r-1)