Difficulty: Easy
Correct Answer: 15 12
Explanation:
Introduction / Context:Here two simple progressions are interleaved: one affects odd-position terms and the other affects even-position terms. Recognizing the interleave is the key step.
Given Data / Assumptions:
Concept / Approach:Separate the series into odd and even indices; examine each subsequence independently.
Step-by-Step Solution:
Odd positions: 1st=8, 3rd=9, 5th=10, 7th=11 → increases by +1.Even positions: 2nd=12, 4th=13, 6th=14 → increases by +1.After the 7th term (11, odd), the 8th term is the next even subsequence value: 14 + 1 = 15.The 9th term resumes the odd subsequence: 11 + 1 = 12.Verification / Alternative check:Write the two subsequences explicitly: odd = 8, 9, 10, 11, 12, … and even = 12, 13, 14, 15, … Interleaving them gives …, 11, 15, 12 which matches the derived pair.
Why Other Options Are Wrong:
Common Pitfalls:Continuing only one subsequence or mixing their orders. Always track position (odd/even) before extending.
Final Answer:15 12
Discussion & Comments