Tính tổng sau S = 4/1.5 + 4/5.9 +...+ 4/2001.2005

* Hướng dẫn giải

$$\begin{gathered} S=\frac{4}{1.5}+\frac{4}{5.9}+\mathrm{...}+\frac{4}{2001.2005} \\ \begin{array}{l}=1-\frac{1}{5}+\frac{1}{5}-\frac{1}{9}+\mathrm{...}+\frac{1}{2001}-\frac{1}{2005}\\ =1-\frac{1}{2005}=\frac{2004}{2005}\end{array} \end{gathered}$$