Find the indicated sum.
-3 + 6 + 9 + . . . + 120
A)4797
B)2460
C)2398
D)4920

Question

Quizplus · Accepted Answer

This is an arithmetic series where the first term $a = 3$, the common difference $d = 3$, and the last term $l = 120$. To find the sum, we first need to find the number of terms $n$. We can use the formula for the $n$th term of an arithmetic sequence, $l = a + (n-1)d$, to find $n$. Plugging in our values gives $120 = 3 + (n-1)3$. Solving for $n$ gives $n = 40$. Then, we use the sum formula for an arithmetic series, $S_n = \frac{n}{2}(a + l)$, to find the sum: $S_{40} = \frac{40}{2}(3 + 120) = 20 \times 123 = 2460$.

Find the Indicated Sum