Đáp án:
$\dfrac{}{}$
Giải thích các bước giải:
$\dfrac{1}{1.2}$ $+$ $\dfrac{1}{2.3}$ $+$ $\dfrac{1}{3.4}$ $+...+$ $\dfrac{1}{9.10}$
$=$ $1$ $-$ $\dfrac{1}{2}$ $+$ $\dfrac{1}{2}$ $-$ $\dfrac{1}{3}$ $+$ $\dfrac{1}{3}$ $-$ $\dfrac{1}{4}$ $+...+$ $\dfrac{1}{9}$ $-$ $\dfrac{1}{10}$
$=$ $1$ $-$ $\dfrac{1}{10}$ = $\dfrac{9}{10}$