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Bunuel
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Bunuel
The infinite sequence \(a_1\), \(a_2\),…, \(a_n\), … is such that \(a_{n} = a_{n-1} + a_{n - 2}\), for all n > 2. What is the value of \(a_1\) ?

(1) \(a_6 = 11\)

(2) \(a_9 = 17\)
Solution:
Pre Analysis:
  • We are given a Fibonacci series and asked the value of its first term \(a_1\)

Statement 1: \(a_6 = 11\)
  • Since \(a_6 = 11\)
  • then we can write \(a_4+a_5=11\)
    \(⇒(a_3+a_2)+(a_4+a_3)=11\)
    \(⇒(a_2+a_1+a_2)+(a_3+a_2+a_2+a_1)=11\)
    \(⇒(2a_2+a_1)+(a_2+a_1+2a_2+a_1)=11\)
    \(⇒5a_2+3a_1=11\)
  • We get one equation with variable \(a_1\) and \(a_2\) which is not enough to get the value of \(a_1\)
  • Thus, statement 1 alone is not sufficient and we can eliminate options A and D

Statement 2: \(a_9 = 17\)
  • Similar to statement 1, we can form an equation with \(a_1\) and \(a_2\)
  • Thus, statement 2 alone is also not sufficient and we can eliminate option B

Combining:
  • We are getting 2 equations of 2 variable each \(a_1\) and \(a_2\)
  • We can solve them simultaneously to get the value of \(a_1\)

Hence the right answer is Option C
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