$\text{Đáp án + Giải thích các bước giải:}$
$\text{B = $\dfrac{1}{4}$ + $\dfrac{1}{5}$ + $\dfrac{1}{6}$ + ... + $\dfrac{1}{19}$ > 1}$
$\text{B = $\dfrac{1}{4}$ + ($\dfrac{1}{5}$ + $\dfrac{1}{6}$ + ... + $\dfrac{1}{9}$) + ($\dfrac{1}{10}$ + $\dfrac{1}{11}$ + ... + $\dfrac{1}{19}$)}$}$
$\text{Vì $\dfrac{1}{5}$ + $\dfrac{1}{6}$ + ... + $\dfrac{1}{9}$ > $\dfrac{1}{9}$ + $\dfrac{1}{9}$ + ... + $\dfrac{1}{9}$ = $\dfrac{5}{9}$ > $\dfrac{1}{2}$}$
$\text{Vì $\dfrac{1}{10}$ + $\dfrac{1}{11}$ + ... + $\dfrac{1}{19}$ > $\dfrac{1}{19}$ + $\dfrac{1}{19}$ + ... + $\dfrac{1}{19}$ = $\dfrac{10}{19}$ > $\dfrac{1}{2}$}$
$\text{=>B > $\dfrac{1}{4}$ + $\dfrac{5}{9}$ + $\dfrac{10}{19}$ > $\dfrac{1}{4}$ + $\dfrac{1}{2}$ + $\dfrac{1}{2}$}$
$\text{=>B > 1}$
$\text{No copy!}$
$\text{#phungducanh97}$
