Question

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?

Answer 1

* The sequence is geometric and the r=-11. Thank you for any help!

Answer 2

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