Đáp án:
Tham khảo
Giải thích các bước giải:
$ B=\dfrac{1}{4}-\dfrac{1}{5}+\dfrac{1}{6}+....+\dfrac{1}{19}
=\dfrac{1}{4}+(\dfrac{1}{5}+\dfrac{1}{6}+....+\dfrac{1}{9})+(\dfrac{1}{10}+\dfrac{1}{1}+....+\dfrac{1}{19})$
$\text{Vì}$$\dfrac{1}{5}+\dfrac{1}{6}+....+\dfrac{1}{9}>\dfrac{1}{9}+\dfrac{1}{9}+\dfrac{1}{9}+...+\dfrac{1}{9}=\dfrac{5}{9}>\dfrac{1}{a}$
$B>\dfrac{1}{4}+\dfrac{5}{9}+\dfrac{10}{9}>\dfrac{1}{4}+\dfrac{1}{2}+\dfrac{1}{2}>1(đpcm)$
