Ta có:
$1 - \dfrac{1}{2} + \dfrac{1}{3} - \dfrac{1}{4} + ... + \dfrac{1}{49} - \dfrac{1}{50}$
$= \left ( 1 + \dfrac{1}{2} + \dfrac{1}{3} + \dfrac{1}{4} + ... + \dfrac{1}{49} + \dfrac{1}{50} \right ) - 2.\left ( \dfrac{1}{2} + \dfrac{1}{4} + \dfrac{1}{6} + ... + \dfrac{1}{48} + \dfrac{1}{50} \right )$
$= \left ( 1 + \dfrac{1}{2} + \dfrac{1}{3} + \dfrac{1}{4} + ... + \dfrac{1}{49} + \dfrac{1}{50} \right ) - \left ( 1 + \dfrac{1}{2} + \dfrac{1}{3} + \dfrac{1}{4} + ... + \dfrac{1}{24} + \dfrac{1}{25} \right )$
$= \dfrac{1}{26} + \dfrac{1}{27} + \dfrac{1}{28} + ... + \dfrac{1}{50}$
