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An arithmetic series sums to n/2 times the sum of the first and last terms. A geometric series sums to a(rⁿ − 1)/(r − 1), reducing to n·a when the ratio is exactly 1.
Arithmetic sum
Sₙ = n/2 · (2a + (n − 1)d) = n/2 · (first + last)
Geometric sum
Sₙ = a(rⁿ − 1) / (r − 1)
10/2 × (6 + 18) = 5 × 24 = 120.
Because evenly spaced terms have a mean equal to the midpoint of the first and last, so the sum is n times that midpoint.