To prove: (23n - 7n -1) is divisible by 49, where n N
Formula used: (a+b)n = nC0an + nC1a n-1b + nC2a n-2b2 + …… +nCn-1abn-1 + nCnbn
(23n - 7n -1) = (23)n – 7n – 1
⇒ 8n - 7n – 1
⇒ (1+7)n – 7n – 1
Now, (23n - 7n -1) = 49K
Therefore (23n - 7n -1) is divisible by 49