Use the formula for the sum of the first n terms of an arithmetic sequence to find the indicated sum.
-$$\sum _ { i = 1 } ^ { 90 } - 5 i$$
A) $- 20,475$
B) $- 20,702$
C) $- 20,247$
D) $- 20,470$

Question

Quizplus · Accepted Answer

The formula for the sum of the first $n$ terms of an arithmetic sequence is $S_n = \frac{n}{2}(a_1 + a_n)$, where $a_1$ is the first term and $a_n$ is the nth term. In this case, $a_1 = -5$ (since when $i = 1$, the term is $-5 \times 1 = -5$), and $a_n = -5 \times 90 = -450$. Plugging these values into the formula gives $S_{90} = \frac{90}{2}(-5 - 450) = 45 \times -455 = -20,475$.

Use the Formula for the Sum of the First N ∑i=190−5i\sum _ { i = 1 } ^ { 90 } - 5 ii=1∑90​−5i

Use the Formula for the Sum of the First N $\sum _ { i = 1 } ^ { 90 } - 5 i$