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Bunuel
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Bunuel
The infinite sequence \(a1, \ a2, \ ..., \ an, \ ...\) is such that \(a_1 = 3, \ a_2 = -1, \ a_3 = 6, a_4 = -2\), and \(a_n = a_{n-4}\) for \(n > 4\). What is the sum of the first 83 terms of the sequence?

A. 120
B. 124
C. 128
D. 132
E. 136
Since \(a_n = a_{n-4}\), then:

\(a_5 = a_1 = 3\)
\(a_6 = a_2 = -1\)
\(a_7 = a_3 = 6\)
\(a_8 = a_4 = -2\)

...

So, the sum of successive blocks of 4 terms is the same: {3, -1, 6, -2} {3, -1, 6, -2} {3, -1, 6, -2} ...

The sum of each block is 3 + (-1) + 6 + (-2) = 6.

Out of the first 83 terms, we have 20 full blocks of {3, -1, 6, -2}, each summing to 6, and then the next three terms are 3 + (-1) + 6 = 8. Therefore, the sum of 83 terms is 20 * 6 + 8 = 128.

Answer: C.

Hope it's clear.

harjas2222
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Bunuel

Since \(a_n = a_{n-4}\), then:

\(a_5 = a_1 = 3\)
\(a_6 = a_2 = -1\)
\(a_7 = a_3 = 6\)
\(a_8 = a_4 = -2\)

...

So, the sum of successive blocks of 4 terms is the same: {3, -1, 6, -2} {3, -1, 6, -2} {3, -1, 6, -2} ...

The sum of each block is 3 + (-1) + 6 + (-2) = 6.

Out of the first 83 terms, we have 20 full blocks of {3, -1, 6, -2}, each summing to 6, and then the next three terms are 3 + (-1) + 6 = 8. Therefore, the sum of 83 terms is 20 * 6 + 8 = 128.

Answer: C.

Hope it's clear.

harjas2222
Thank you man, appreciated
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