SAT Math Multiple Choice Practice Question 345: Answer and Explanation

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Question: 345

15. -1, 1, 2,-1, 1, 2,-1, 1, 2, …

The sequence above continues according to the pattern shown. What is the sum of the first 25 terms of this sequence?

A. 15
B. 16
C. 18
D. 19
E. 21

Correct Answer: A


A Find the sum of each repetition of the pattern:

Next, determine how many times the pattern repeats in the first 25 terms: with a remainder of 1.

Multiply the sum of the pattern by 8 to obtain the sum of the first 24 terms:

The 25th term is 1, which makes the sum .

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