$\dfrac{1}{1 × 2}$ + $\dfrac{1}{2 × 3}$ + $\dfrac{1}{3 × 4}$ + ... + $\dfrac{1}{999 × 1000}$
= $\dfrac{1}{1}$ - $\dfrac{1}{2}$ + $\dfrac{1}{2}$ - $\dfrac{1}{3}$ + $\dfrac{1}{3}$ - $\dfrac{1}{4}$ + .... + $\dfrac{1}{999}$ - $\dfrac{1}{1000}$
= $\dfrac{1}{1}$ - $\dfrac{1}{1000}$
= $\dfrac{1000}{1000}$ - $\dfrac{1}{1000}$
= $\dfrac{999}{1000}$
$\text{#nocopy}$
$\text{@huyen20009a5a4}$