FM June 2024 p11 q02
4158
Prove by mathematical induction that \(6^{4n} + 38^n - 2\) is divisible by 74 for all positive integers \(n\). [6]
Solution
Base Case: For \(n = 1\), \(6^4 + 38^1 - 2 = 1332\), which is divisible by 74.
Inductive Step: Assume \(6^{4k} + 38^k - 2\) is divisible by 74 for some positive integer \(k\).
Then, consider \(6^{4(k+1)} + 38^{k+1} - 2\):
\(= (1295 + 1) \times 6^{4k} + (37 + 1) \times 38^k - 2\)
This expression is divisible by 74 because \(1295 \times 6^{4k} + 37 \times 38^k\) is divisible by 74.
Hence, by induction, \(6^{4n} + 38^n - 2\) is divisible by 74 for every positive integer \(n\).
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