(1 + 1) × 1 / 2 = 1, (1 + 1) × 2 / 2, (1 + 3) × 3 / 2, (1 + 4) × 4 / 2?

(1 + 1) × 1 / 2 = 1, (1 + 1) × 2 / 2, (1 + 3) × 3 / 2, (1 + 4) × 4 / 2?

(1 + n) * n / 2, n is a natural number