### Home > CCA2 > Chapter Ch10 > Lesson 10.2.1 > Problem10-89

10-89.

Consider the series $8 + 1 + (−6) + (−13) + … + (-90)$.

1. How many terms are there?

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

$15$

1. What is the sum?

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