Question

Find the 9th partial sum of the geometric sequence an=43n–1.

Answer 1

39364.

Explanation:

I guess its an = 4*(3)^(n-1)

r = 3, a1 = 4 and n = 9

Sum of n terms = a1 * (r^n - 1) / (r - 1)

so S9 = 4 *(3^9 - 1) / (3 - 1)

= 4 * (19683 - 1) / 2

= 78728/2

= 39364.