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Prove that for every integer n (...-3, -2, -1, 0, 1, 2, 3, 4...), the expression n2 + n will always be even.
Defination : An integer n is said to be even if n = 2k for some integer k and an integer m is said to be odd if m=2k+1 for some integer k .
Suppose n is given integer .
n is either even or odd .
Case 1 : If n is even .
Then n = 2k for some integer k . Now ,
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=
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So is even as it is multiple of 2 .
Case 2 : If n is odd .
Then n = 2k +1 for some integer k . Now ,
=
=
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So is even as it is multiple of 2 .
i.e., n is even or odd is always even .
Hence is even for every integer n .
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