Question

I have NO idea how to get the answer to this! I've used the same formula for similar problems and got the right answer, but why isn't it working for this problem?! Here's the question:

Find the sum of the first n terms of the sequence.
5, 20, 80, ... ; n = 9

Here's my choices:
A) 349525
B) 436905
C) - 1048575
D) 1747625

This is what I did:
Sn=n/2[2a+(n-1)d]
S9=9/2[2(5)+(9-1)(20-5)]
S9=9/2[10+8(15)]
S9=9/2(10+120)
S9=9/2(130)
S9=585

Please Help! What am I doing wrong?

Answer 1

B. 436905

Explanation:

This is a Geometric sequence with:

a = 5

r = 20/5 = 4

S9 = 5[4⁹ - 1]/(4 - 1)

5(262143)/3

1310715/3

436905

Answer 2

B) 436905

Explanation:

This is a geometric sequence , not an arithmetic sequence

We multiply by 4 each time

r = 2nd term/ 1st term = 20/5 =4

We want the sum of the first 5 terms

a1(1−r^n)

Sn= ------------------ where a1 is the first term

1−r

5(1−4^9)

Sn= ------------------ where a1 is the first term

1−4

Sn = (-1310715)

-----------------

-3

Sn = 436905