$M$ = $\dfrac{2}{2.3.4}$ +$\dfrac{2}{3.4.5}$ +...+$\dfrac{2}{19.20.21}$
$M$ = $\dfrac{1}{2.3}$ - $\dfrac{1}{3.4}$ + $\dfrac{1}{3.4}$ - $\dfrac{1}{4.5}$ +...+ $\dfrac{1}{19.20}$- $\dfrac{1}{20.21}$
$M$ = $\dfrac{1}{2.3}$-$\dfrac{1}{20.21}$
$M$ = $\dfrac{1}{6}$-$\dfrac{1}{420}$
$M$ = $\dfrac{70}{420}$ - $\dfrac{1}{420}$
$M$ = $\dfrac{69}{420}$ = $\dfrac{23}{140}$
$Vậy$ $M$ = $\dfrac{23}{140}$
$chúc$ $bạn$ $học$ $tốt$ $nhé$