isxyei
(n+k)^2 = n^2 + 2nk +k^2 = 1 + n^2 + 2nk +k^2 -1=
= 1 + n^2 + (k+1+k-1)n + (k+1)(k-1) = 1 + (n+k+1)(n+k-1)
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