Đáp án+Giải thích các bước giải:
5.
$B=\dfrac{1}{4}+\dfrac{1}{5}+\dfrac{1}{6}+...........+\dfrac{1}{19}$
$B=\dfrac{1}{4}+\left(\dfrac{1}{5}+\dfrac{1}{6}+.......+\dfrac{1}{19}\right)$
Nhận thấy:
$\dfrac{1}{5}>\dfrac{1}{20}$
$\dfrac{1}{6}>\dfrac{1}{20}$
....................
$\dfrac{1}{19}>\dfrac{1}{20}$
$\Rightarrow B>\dfrac{1}{4}+\left(\dfrac{1}{20}+\dfrac{1}{20}+.........+\dfrac{1}{20}\right)$
$B>\dfrac{1}{4}+\dfrac{1}{20}.15$
$B>\dfrac{1}{4}+\dfrac{3}{4}=1\Rightarrow B>1\(đpcm)$
