Pre-Calculus/Honors Algebra 2 Question?

Question

Find the partial sum (S sub n) for a geometric series such that a sub 4= 40, a sub 9=1280, and n=9

Answer 1

The fourth term in geometric series is a₄ (a r³ ) = 40 .

The ninth term in geometric series is a₉  (a r⁸ )  = 1280 .

The common ratio r is 

Now put r = 2 in foutrh term , to find  value of a .

a 2³ = 40 

a (8) = 40

a = 40 / 8

a = 5

The sum of n terms in geometric series is   .

So the sum of 9 terms in geometric series is 

So the  sum of 9 terms in geometric series  is 2555 .