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Calculate the sum of the first 20 terms of an arithmetic sequence with first term 18 and common difference 11.

Guest Jan 15, 2020

geno3141 · Answer 1 · 2020-01-15T02:05:28

Let n = number of terms

Let t₁ = the first term

Let t_n = the n^th term

Let d = the common difference

Let S = the sum of the terms

Two formulas: S = n · (t₁ + t_n) / 2 t_n = t₁ + (n - 1) · d

t_n = t₁ + (n - 1) · d ---> t_n = 18 + (20 - 1) · 11 ---> 227

S = n · (t₁ + t_n) / 2 ---> S = 20 · (18 + 227) / 2 ---> 2450