Tính tổng 1 / 2.3 + 1 / 3.4 + 1/ 4.5 + ... + 1 /19.20

* Hướng dẫn giải

$\begin{array}{l}\frac{1}{2.3}+\frac{1}{3.4}+\frac{1}{4.5}+\mathrm{...}+\frac{1}{19.20}=\frac{1}{2}-\frac{1}{3}+\frac{1}{3}-\frac{1}{4}+\frac{1}{4}-\frac{1}{5}+\mathrm{...}+\frac{1}{19}-\frac{1}{20}\\ \frac{1}{2}-\frac{1}{20}=\frac{9}{20}\end{array}$