Sum of AP: Sₙ = n/2 × (2a + (n-1)d) or Sₙ = n/2 × (first + last) = n/2 × (a + aₙ). Formula 2 more practical when last term known. Example: Sum of 1+2+3+...+100 = 100/2 × (1+100) = 5050. Or using formula 1: a=1, d=1, n=100. Sₙ = 100/2 × (2 + 99) = 5050. Solving: (1) Identify a, d, n; (2) Choose appropriate formula; (3) Calculate. For finding n given Sₙ: Rearrange quadratic equation. Shortcut: Use formula 2 when last term obvious. Exam tip: Double-check if sum is from first term or middle term. Verify using simple cases (n=2 or n=3).