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Category: Algebra

1-1-2-1-2-3-1-19-20-1-20-21-

Question Number 163849 by alephzero last updated on 11/Jan/22 112+123+11920+12021=? Answered by cortano1 last updated on 11/Jan/22 $$\:\underset{{k}=\mathrm{1}} {\overset{\mathrm{20}} {\sum}}\:\frac{\mathrm{1}}{{k}\left({k}+\mathrm{1}\right)}\:=\:\underset{{k}=\mathrm{1}} {\overset{\mathrm{20}} {\sum}}\:\left(\frac{\mathrm{1}}{{k}}−\frac{\mathrm{1}}{{k}+\mathrm{1}}\right) \