We need to find the value of the infinite sum $\sum_{k=2}^{\infty} (\frac{1}{k} - \frac{1}{k-1})$.

AnalysisInfinite SeriesTelescoping SeriesLimits
2025/3/6

1. Problem Description

We need to find the value of the infinite sum k=2(1k1k1)\sum_{k=2}^{\infty} (\frac{1}{k} - \frac{1}{k-1}).

2. Solution Steps

The given sum is a telescoping sum. Let's write out the first few terms to see the pattern.
Sn=k=2n(1k1k1)=(1211)+(1312)+(1413)++(1n1n1)S_n = \sum_{k=2}^{n} (\frac{1}{k} - \frac{1}{k-1}) = (\frac{1}{2} - \frac{1}{1}) + (\frac{1}{3} - \frac{1}{2}) + (\frac{1}{4} - \frac{1}{3}) + \dots + (\frac{1}{n} - \frac{1}{n-1}).
We observe that most of the terms cancel out. Specifically, the 12\frac{1}{2} term cancels with the 12-\frac{1}{2} term, the 13\frac{1}{3} term cancels with the 13-\frac{1}{3} term, and so on. The only terms that remain are the first term 11-\frac{1}{1} and the last term 1n\frac{1}{n}.
Thus, the partial sum SnS_n is given by:
Sn=1n1S_n = \frac{1}{n} - 1
Now, to find the value of the infinite sum, we need to find the limit of the partial sum SnS_n as nn approaches infinity.
k=2(1k1k1)=limnSn=limn(1n1)\sum_{k=2}^{\infty} (\frac{1}{k} - \frac{1}{k-1}) = \lim_{n \to \infty} S_n = \lim_{n \to \infty} (\frac{1}{n} - 1)
Since limn1n=0\lim_{n \to \infty} \frac{1}{n} = 0, we have
limn(1n1)=01=1\lim_{n \to \infty} (\frac{1}{n} - 1) = 0 - 1 = -1

3. Final Answer

1-1

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