Question

What is the sum of a 7-term geometric series if the first term is −6, the last term is −24,576, and the common ratio is 4?

Answer 1

sn = a1(1 - r^n) / (1 - r)

n = the sum of n terms = 7
a1 = first term = -6
r = common ratio = 4
now we sub ...
s7 = -6(1 - 4^7) / (1 - 4)
s7 = -6(1 - 16384) / -3
s7 = -6( - 16383) / -3
s7 = 98298/-3
s7 = - 32766

Answer 2

this should help...
http://www.regentsprep.org/Regents/math/algtrig/ATP2/GeoSeq.htm