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f(n) = (n² + n) / 2

f(1) = (1² + 1) / 2 = 1

f(2) = (2² + 2) / 2 = 3

f(3) = (3² + 3) / 2 = 6

f(4) = (4² + 4) / 2 = 10

f(5) = (5² + 5) / 2 = 15

f(6) = (6² + 6) / 2 = 21

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11y ago

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