Đáp án:
Ta có :
$\dfrac{1}{30}$ + $\dfrac{1}{42}$ + $\dfrac{1}{56}$ + .... + $\dfrac{1}{9900}$
= $\dfrac{1}{5.6}$ + $\dfrac{1}{6.7}$ + $\dfrac{1}{7.8}$ + .... + $\dfrac{1}{99.100}$
= $\dfrac{1}{5}$ - $\dfrac{1}{6}$ + $\dfrac{1}{6}$ - $\dfrac{1}{7}$ + .... + $\dfrac{1}{99}$ - $\dfrac{1}{100}$
= $\dfrac{1}{5}$ - $\dfrac{1}{100}$
= $\dfrac{19}{100}$
Giải thích các bước giải:
