Posted by **Brock** on Tuesday, July 10, 2012 at 9:58pm.

Find the sum of the first n terms of the sequence. The sequences are either arithmetic or geometric.

13. -1, 11, -121, ...; n = 9

A. 235,794,769

B. 429,981,696

C. -429,981,696

D. 196,495,641

E. -196,495,641

F. -235,794,769

I know that n = # of terms (9)

d = -11

a = -1

formula: n/-11(a + last term)

9/-11(-1 +?)

Also term 4 = 1331

But, the last term is too large (-214358881).

Am I doing something wrong?

- AP Calculus -
**Brock**, Tuesday, July 10, 2012 at 10:10pm
* The sequence is geometric and the r=-11. Thank you for any help!

- AP Calculus -
**Reiny**, Tuesday, July 10, 2012 at 10:46pm
yes, you realized that the series was geometric rather than arithmetic

so you have

a=-1, r = -11, n = 9

you now need the formula for the sum of a geometric, not arithmetic like you stated.

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

= -1( (-11)^9 - 1)/(-11-1)

= -196495641

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