Prove that 5^40-5^39 is divisible by 4
540 = 5 * 539
5 * 539 - 539 =
4 * 539 which is obviously divisible by '4' 4* x is divisiable by 4 (x is 539 in this case)
$5^{40} - 5^{39} = 5^{39}(5 - 1) = 5^{39}(4)$
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