Find the sum of a 12–term arithmetic sequence where the last term is 13 and the common difference is –10

Frances Pham

Frances Pham

Answered question

2022-11-08

Find the sum of a 12–term arithmetic sequence where the last term is 13 and the common difference is –10

Answer & Explanation

mainzollbtt

mainzollbtt

Beginner2022-11-09Added 13 answers

Let's first determine the first term.
t n = a + ( n - 1 ) d
13 = a + ( 12 - 1 ) - 10
13 = a - 110
123 = a
We now apply the formula s n = n 2 ( t 1 + t n ) .
s 12 = 12 2 ( 123 + 13 )
s 12 = 6 ( 136 )
s 12 = 816

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