√(9·n^2 + 2·n + 1) - 3·n
= (√(9·n^2 + 2·n + 1) - 3·n) * (√(9·n^2 + 2·n + 1) + 3·n) / (√(9·n^2 + 2·n + 1) + 3·n)
= (9·n^2 + 2·n + 1 - 9·n^2) / (√(9·n^2 + 2·n + 1) + 3·n)
= n·(2 + 1/n) / (n·√(9 + 2/n + 1/n^2) + 3·n)
= (2 + 1/n) / (√(9 + 2/n + 1/n^2) + 3)
lim n --> ∞
= (2 + 0) / (√(9 + 0 + 0) + 3) = 2 / (3 + 3) = 1/3