Question

show that only one of numbers n,n+2,n+4 is divisible by 4.

Answer 1

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Let n = 3k, 3k + 1 or 3k + 2.

(i) When n = 3k:

n is divisible by 3.

n + 2 = 3k + 2

》 n + 2 is not divisible by 3.

n + 4 = 3k + 4 = 3(k + 1) + 1

》 n + 4 is not divisible by 3.

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(ii) When n = 3k + 1:

n is not divisible by 3.

n + 2 = (3k + 1) + 2 = 3k + 3 = 3(k + 1)

》 n + 2 is divisible by 3.

n + 4 = (3k + 1) + 4 = 3k + 5 = 3(k + 1) + 2

》 n + 4 is not divisible by 3.

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(iii) When n = 3k + 2:

n is not divisible by 3.

n + 2 = (3k + 2) + 2 = 3k + 4 = 3(k + 1) + 1

》 n + 2 is not divisible by 3.

n + 4 = (3k + 2) + 4 = 3k + 6 = 3(k + 2)

》 n + 4 is divisible by 3.

Hence, exactly one of the numbers n, n + 2 or n + 4 is divisible by 3.