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10-89.
1. Consider the series 8 + 1 + (−6) + (−13) + … + (−90). Homework Help ✎

1. How many terms are there?

2. What is the sum?

Write an expression for the sequence, and find n when t(n) = −90.

15

$\text{Use the summation formula }S_n=\frac{n}{2}\left(t\left(1\right)+t\left(n\right)\right)$