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Convergent Infinite Geometric Series

An infinite geometric series is convergent when the absolute value of its common ratio satisfies r<1|r| < 1. In a convergent series, each successive term is smaller than the one before it, so the partial sums grow more and more slowly and approach a fixed limiting value rather than increasing without bound.

For example, the series 12+14+18+116+132+164+\frac{1}{2} + \frac{1}{4} + \frac{1}{8} + \frac{1}{16} + \frac{1}{32} + \frac{1}{64} + \dots has a1=12a_1 = \frac{1}{2} and r=12r = \frac{1}{2}. Since r=12<1|r| = \frac{1}{2} < 1, the series is convergent. Computing a few partial sums confirms this: S100.9990S_{10} \approx 0.9990, S200.9999990S_{20} \approx 0.9999990, and S300.9999999991S_{30} \approx 0.9999999991. Each partial sum is closer to 11 than the one before it. Because r<1|r| < 1, the expression rnr^n shrinks toward zero as nn grows, and a finite sum can be determined using the formula for the sum of an infinite geometric series.

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Updated 2026-05-25

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