Question

Given a GPS with a=729 and 7th term=64, determine S7

Answer 1

Given that

a = 729

T7 =64

We know that

Tn = ar^n-1

T7 =729r^7-1=64

729r^6 = 64

r^6 = 64/729

r^6 =(2/3)^6

r = 2/3

we know that

Sn = a ( 1 - r^n)/(1 - r)

Sn = 729 ( 1 - 2/3^6 )/(1 - 2/3)

Sn = 729 ( 1 - 128/2187)/1/3

Sn = 729.3/1 ( 2187 -128/2187)

Sn = 2059