Tính nhanh: \(\frac{1}{{1 \times 2}} + \frac{1}{{2 \times 3}} + \frac{1}{{3 \times 4}} + ... + \frac{1}{{2021 \times
Tính nhanh: \(\frac{1}{{1 \times 2}} + \frac{1}{{2 \times 3}} + \frac{1}{{3 \times 4}} + ... + \frac{1}{{2021 \times 2022}}\).
Đáp án đúng là: C
Áp dụng công thức: \(\frac{1}{{n \times (n + 1)}}\,\, = \,\,\frac{1}{n}\,\, - \,\,\frac{1}{{n\, + \,1}}\).
\(\begin{array}{l}\frac{1}{{1\, \times \,2}}\,\, + \,\,\frac{1}{{2 \times 3}}\,\, + \,\,\frac{1}{{3\, \times \,4}}\,\, + \,\,...\,\, + \,\,\frac{1}{{2021\,\, \times \,\,2022}}\\ = \,\,\frac{1}{1}\,\, - \,\,\frac{1}{2}\,\, + \,\,\frac{1}{2}\,\, - \,\,\frac{1}{3}\,\, + \,\,\frac{1}{3}\,\, - \,\,\frac{1}{4}\,\, + \,\,...\,\, + \,\,\frac{1}{{2021}}\,\, - \,\,\frac{1}{{2022}}\\ = 1\,\, - \,\,\frac{1}{{2022}}\\ = \,\,\frac{{2021}}{{2022}}\end{array}\)
Đáp số: \(\frac{{2021}}{{2022}}\).
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