Question

For the geometric sequence

-1,-2,-4... find a8

Answer 1

Geometric sequence formula gn = g1 (r)^n - 1

To find the ratio you divide g2 by g1

G1 = first number in the series



To find g8 you just plug 8 instead of n

g8 = g1 (r)^8 - 1

g8 = -1 (2)^8 - 1

g8 = -128

Answer 2

an=a1(r)^(n-1)
a1=first term
r=common ratio

so
firs term is -1

each is double the prevoius
r=2

an=-1(2)^n-1
8th term
a8=-1(2)^(8-1)
a8=-1(2)^7
a8=-1(128)
a8=-128
the 8th term is -128