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Evaluate the geometric series given by the sum of 3(2^k) for k = 1 to 5.
A378
B186
C93
D126
Answer & Solution
Correct answer: B. 186
1. Find the first term by substituting k = 1: a_1 = 3 x 2 = 6, and the ratio is r = 2.
2. There are n = 5 terms, so use S_n = a_1(1 - r^n)/(1 - r).
3. S_5 = 6(1 - 2^5)/(1 - 2) = 6(1 - 32)/(-1).
4. S_5 = 6 x 31 = 186, which matches adding 6 + 12 + 24 + 48 + 96 directly.
5. Option A, 378, runs the sum through k = 6, one term too far.
_Source: OpenStax Algebra and Trigonometry (CC BY 4.0), Ch 13 "Sequences, Probability and Counting Theory", section 13.4 SERIES AND THEIR NOTATIONS_
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