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)/(r1)
= 1( (11)^9  1)/(111)
= 196495641
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