Use the formula for S n to find the sum of the first five terms of the geometric sequence.
-5, 20, 80, 320, . . .
A) 1707
B) 1705
C) 1701
D) 1715

Question

Quizplus · Accepted Answer

The formula for the sum of the first $n$ terms of a geometric sequence is $S_n = \frac{a(1 - r^n)}{1 - r}$, where $a$ is the first term, $r$ is the common ratio, and $n$ is the number of terms. Here, $a = 5$, $r = 4$ (since each term is 4 times the previous one), and $n = 5$. Plugging these values into the formula gives $S_5 = \frac{5(1 - 4^5)}{1 - 4} = \frac{5(1 - 1024)}{-3} = \frac{5(-1023)}{-3} = 1705$.

Use the Formula for Sn to Find the Sum of the First

Use the Formula for S_n to Find the Sum of the First