Sum of GP: Sₙ = a(1 - r^n)/(1 - r) if r ≠ 1; Sₙ = na if r = 1. For infinite GP with |r| < 1: S∞ = a/(1 - r). Example: 2 + 6 + 18 + 54 (4 terms). Sₙ = 2(1 - 3⁴)/(1 - 3) = 2(1 - 81)/(-2) = 80. Infinite series: 1 + 1/2 + 1/4 + ... = 1/(1 - 1/2) = 2. Solving: (1) Identify a, r, n; (2) Choose formula; (3) Calculate. For divergent series (|r| ≥ 1, r ≠ 1): Sum is infinite or undefined. Shortcut: For r close to 1, S∞ ≈ na/2. Exam tip: Check convergence (|r| < 1) before calculating infinite sum. Verify using partial sums.