tiính nhanh : 2/3 2/15 2/35
Giải thích
\(\frac{2}{3} + \frac{2}{{15}} + \frac{2}{{35}} + \frac{2}{{63}}\)
\(\begin{array}{l} = \frac{2}{{1.3}} + \frac{2}{{3.5}} + \frac{2}{{5.7}} + \frac{2}{{7.9}}\\ = 1 - \frac{1}{3} + \frac{1}{3} - \frac{1}{5} + \frac{1}{5} - \frac{1}{7} + \frac{1}{7} - \frac{1}{9}\\ = 1 - \frac{1}{9}\\ = \frac{8}{9}\end{array}\)