Formulas
Arithmetic: aₙ = a₁ + (n − 1) × d, sum = (a₁ + aₙ) × n ÷ 2. Geometric: aₙ = a₁ × rⁿ⁻¹, sum = a₁ × (rⁿ − 1) ÷ (r − 1). If −1 < r < 1, the infinite sum is a₁ ÷ (1 − r).
Example
2, 5, 8, 11…: the 10th term is 2 + 9 × 3 = 29 and the first 10 terms add up to 155.
Frequently asked questions
What is the difference?
An arithmetic sequence adds the same number each time; a geometric one multiplies by the same number.
See also
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