Tính nhanh: (1/2 - 1) (1/3 - 1) (1/4 - 1) ..... (1/1999 - 1)

* Hướng dẫn giải

$$\begin{gathered} \left(\frac{1}{2}-1\right)\left(\frac{1}{3}-1\right)\left(\frac{1}{4}-1\right)\mathrm{.....}\left(\frac{1}{1999}-1\right) \\ =\frac{-1}{2}.\frac{-2}{3}.\frac{-3}{4}\mathrm{....}\frac{-1998}{1999} \\ =\frac{1}{1999} \end{gathered}$$