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Prove the divisibility of the following numbers by 7


5 {}^(5) - 5 {}^(4) + 5 {} {}^(3) \:

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5^5 mod 7 = 3
5^4 mod 7 = 2
5^3 mod 7 = 6

3 -2 +6 mod 7 = 0
Therefore, 5^5 -5^4 +5^3 mod 7 = 0 and that sum is divisible by 7.
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