Use the formula for $\mathbf{S}_{\mathbf{n}}$ to find the sum for the geometric sequence.
-$1,3,9,27,81$
A) 122
B) 118
C) 121
D) 111

Question

Quizplus · Accepted Answer

The sum of a geometric sequence can be found using the formula $S_n = \frac{a(1 - r^n)}{1 - r}$, where $a$ is the first term, $r$ is the common ratio, and $n$ is the number of terms. Here, $a = 1$, $r = 3$, and $n = 5$. Plugging these values into the formula gives $S_5 = \frac{1(1 - 3^5)}{1 - 3} = \frac{1 - 243}{-2} = 121$.

Use the Formula For Sn\mathbf{S}_{\mathbf{n}}Sn​ To Find the Sum for the Geometric Sequence

Use the Formula For $\mathbf{S}_{\mathbf{n}}$ To Find the Sum for the Geometric Sequence