tính nhanh 1/15 1/21 1/28 1/36 1/45

Lời giải:

\(\begin{array}{l}B:2 = \frac{1}{{30}} + \frac{1}{{42}} + \frac{1}{{56}} + \frac{1}{{72}} + \frac{1}{{90}} + \frac{1}{{110}}\\B:2 = \frac{1}{{5.6}} + \frac{1}{{6.7}} + \frac{1}{{7.8}} + \frac{1}{{8.9}} + \frac{1}{{9.10}} + \frac{1}{{10.11}}\\B:2 = \frac{1}{5} - \frac{1}{6} + \frac{1}{6} - \frac{1}{7} + \frac{1}{7} - \frac{1}{8} + \frac{1}{8} - \frac{1}{9} + \frac{1}{9} - \frac{1}{{10}} + \frac{1}{{10}} - \frac{1}{{11}}\\B:2 = \frac{1}{5} - \frac{1}{{11}}\\B:2 = \frac{6}{{55}}\\B = \frac{{12}}{{55}}\end{array}\)