解答
$a_n = a_1 \cdot r^{n-1}$; Infinite sum $S_\infty = \frac{a_1}{1 - r}$ for $|r| < 1$.
解説
If $|r| \geq 1$, the infinite series diverges and has no finite sum.
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What is the formula for the $n$-th term of a geometric sequence, and the sum of an infinite geometric series ($|r| < 1$)?
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