Q. 93.9( 23 Votes )

Prove that the square of any positive integer is of the form 5q, 5q + 1, 5q + 4 for some integer q.

Answer :

Since any positive integer n is of the form 5p or 5p + 1, or 5p + 2 or 5m + 3 or 5p +4.

When n = 5p, then


n2 = (5p)2 = 25p2 5 (5p2) = 5a, where a =5p2


When n = 5p + 1,


then n2 = (5p + 1)2 = 25p2 + 10p + 1


5p(5p + 2) + 1 5a + 1 Where a = p(5p + 2)


When n = 5p + 2,


then n2 = (5p + 2)2 25p2 + 20p + 4 5p(5p + 4) + 4


5a + 4 where a = p(5p + 4)


When n = 5p + 3, then n2 = 25p2 + 30p + 9


5(5p2 + 6p + 1) + 4 5a + 4 where a = 5p2 + 6m + 1


When n = 5p + 4, then n2 = (5p + 4)2 = 25p2 + 40p + 16 5(5p2 + 8m + 3) + 1 = 5a + 1 where a = 5p2 + 8m + 3


Therefore from above results we got that n2 is of the form 5q or, 5q + 1 or, 5q + 4.


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