Sum of an AP
Sequences, series and the binomial theorem · Mathematics
Study notes
Q: Find the sum of the first 25 terms of 3, 7, 11, ... Also find the sum of 1 + 2 + ... + 50. Part A: a = 3, d = 4, n = 25. S₂₅ = 25/2·(6 + 24×4) = 25/2·102 = 25 × 51 = 1275. Part B: Sum 1 to 50 = 50×51/2 = 1275. (Same answer - coincidence! Both are valid.)