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5^n -1 is divisible by 4....

`5^n -1` is divisible by 4.

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Let `P_n = 5^n -1`
When n = 1,
` 5^1 - 1 = 4` is divisible by 4. `therefore P_1` is true.
Let `P_k` be true i.e,
`5^k -1` = 4m, m in Z Now `5^(k+1)-1 = 5^k . 5 - 5 + 4 = 5(5^k -1) + 4`
`5 xx 4m + 4 = 4(5m + 1)` which is divisible by 4.
`therefore P_(k+1)` is true.
`therefore P_n` is true for all values of `n in N`
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