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In the Fibonacci sequence each term is the sum of the two preceding terms. The eighth term is 21 and the ninth term is 34. What is the eleventh term?
A55
B76
C68
D89
Answer & Solution
Correct answer: D. 89
1. Each Fibonacci term adds the two terms just before it.
2. The tenth term is the ninth plus the eighth: 34 + 21 = 55.
3. The eleventh term is the tenth plus the ninth: 55 + 34 = 89.
4. So the answer requires two applications of the rule, not one.
5. Option A, 55, stops one step early, giving the tenth term instead of the eleventh.
_Source: OpenStax Algebra and Trigonometry (CC BY 4.0), Ch 13 "Sequences, Probability and Counting Theory", section 13.1 SEQUENCES AND THEIR NOTATIONS_
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