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f(n) = ( n + 2)^2 + n^2 - 4
f(n) = (n + 2) (n + 2) + n^2 - 4
f(n) = n^2 + 4n + 4 + n^2 - 4
f(n) = 2n^2 + 4n
f(n) = n(2n + 4)