Giải thích các bước giải:
Ta có :
$\dfrac{91}{1.4}+\dfrac{91}{4.7}+\dfrac{91}{7.10}+...+\dfrac{91}{88.91}$
$=91\left(\dfrac{1}{1.4}+\dfrac{1}{4.7}+\dfrac{1}{7.10}+...+\dfrac{1}{88.91}\right)$
$=\dfrac{91}3\left(\dfrac{3}{1.4}+\dfrac{3}{4.7}+\dfrac{3}{7.10}+...+\dfrac{3}{88.91}\right)$
$=\dfrac{91}{3}\left(\dfrac{4-1}{1.4}+\dfrac{7-4}{4.7}+\dfrac{10-7}{7.10}+...+\dfrac{91-88}{91.88}\right)$
$=\dfrac{91}{3}\left(\dfrac11-\dfrac14+\dfrac14-\dfrac17+\dfrac17-\dfrac1{10}+...+\dfrac1{88}-\dfrac1{91}\right)$
$=\dfrac{91}{3}\left(1-\dfrac1{91}\right)$
$=30$
