Prove that the square of any positive integer of the form 5q + 1 is of the same form.

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2 Answers

Let n = 5q + 1 where q is a positive integer
∴ n2 = (5q + 1)2
= 25q2 + 10q + 1
= 5(5q2 + 2q) + 1
= 5m + 1, where m is some integer
Hence, the square of any positive integer of the form 5q + 1 is of the same form.

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Let N = 5p + 1.

Then,

According to the condition:

N2 = 25p2 + 10p + 1 ⇒ 5(5p2 + 2p) + 1 ⇒ 5A+1

Where A = 5p2 + 2p

Therefore N2 is of the form 5m + 1.

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