Find the sum of the first 9 terms of the sequence.

Question

Show all work. 2, -7, -16, -25, ...

Answer 1

So like the other question I answered, the formula for your arithmetic sequence is a_n = 2-9(n-1).

 

The formula for the sum of an arithmetic sequence is S_n = n(a₁ + a_n) / 2

 

So your 9th term is a₉ = 2-9(9-1) = -70

 

So S₉ = 9(2+ -70) / 2 = -306.

 

Hope this helped.