Giải thích các bước giải:
Ta có:
$B=\dfrac14+\dfrac15+\dfrac16+...+\dfrac1{19}$
$\to B=\dfrac14+(\dfrac15+\dfrac16+...+\dfrac19)+(\dfrac1{10}+..+\dfrac1{19})$
$\to B>\dfrac14+(\dfrac1{10}+\dfrac1{10}+...+\dfrac1{10})+(\dfrac1{20}+...+\dfrac1{20})$
$\to B>\dfrac14+\dfrac5{10}+\dfrac{10}{20}$
$\to B>\dfrac14+\dfrac12+\dfrac12$
$\to B>\dfrac12+\dfrac12$
$\to B>1$
