$\dfrac{1}{1.2}$+$\dfrac{1}{2.3}$+$\dfrac{1}{3.4}$+...+ $\dfrac{1}{999.1000}$
=1-$\dfrac{1}{2}$+$\dfrac{1}{2}$-$\dfrac{1}{3}$+$\dfrac{1}{3}$-$\dfrac{1}{4}$+....+$\dfrac{1}{999}$-$\dfrac{1}{1000}$
=1-$\dfrac{1}{1000}$
=$\dfrac{999}{1000}$
Chúc học tốt!!