Đáp án:
Để ý nhé: Có $\frac{1}{1×2}=1-\frac{1}{2}$ ; $\frac{1}{2×3}=\frac{1}{2}-\frac{1}{3}$ ....
=> $\frac{1}{1×2} + \frac{1}{2×3}+ \frac{1}{3×4} + ... + \frac{1}{9×10}$
= $1-\frac{1}{2} + \frac{1}{2}-\frac{1}{3} + \frac{1}{3}-\frac{1}{4} + ... + \frac{1}{9}-\frac{1}{10}$
= $1- \frac{1}{10} = \frac{9}{10}$