Tính hợp lí: D = 1/2 + 1/14 + 1/35 + 1/65 + 1/104 + 1/152

* Hướng dẫn giải

$D=\frac{2}{4}+\frac{2}{28}+\frac{2}{70}+\frac{2}{130}+\frac{2}{208}+\frac{2}{304}$

$=\frac{2}{3}\left(\frac{3}{1.4}+\frac{3}{4.7}+\frac{3}{7.10}+\frac{3}{10.13}+\frac{3}{13.16}+\frac{3}{16.19}\right)=\frac{12}{19}$