Tính nhanh :5.18-10.27+15.36/10.36-20.54+30.72

* Hướng dẫn giải

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