Saturday, October 30, 2021

For every \(n\in\mathbb{N}\), \[\sum_{k=1}^{n} \frac{1}{k(k+1)}=1-\frac{1}{n+1}\]

Base case: \(n=1\) \[\frac{1}{1\cdot2}=\frac{1}{2}\] \[1-\frac{1}{1+1}=1-\frac{1}{2}=\frac{1}{2}\] So \(\sum_{k=1}^{n} \frac{1}{k(k+1)}=1-\frac{1}{n+1}\) for \(n=1\)

Induction step: \[\begin{align} \sum_{k=1}^{n+1} \frac{1}{k(k+1)} &= \sum_{k=1}^{n} \frac{1}{k(k+1)}+\frac{1}{(n+1)(n+2)}\\ &= 1-\frac{1}{n+1}+\frac{1}{(n+1)(n+2)}\\ &= 1-\frac{n+2-1}{(n+1)(n+2)}\\ &= 1-\frac{1}{n+2} \end{align}\]

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