Đáp án:
Giải thích các bước giải:
1. Đề bài
$B= \dfrac{1}{4}+ ( \dfrac{1}{5}+ \dfrac{1}{6}+...+ \dfrac{1}{9})+ ( \dfrac{1}{10}+ \dfrac{1}{11}...+ \dfrac{1}{19})$
Vì $\dfrac{1}{5}+ \dfrac{1}{6}+...+ \dfrac{1}{9}> \dfrac{1}{9}+ \dfrac{1}{9}+...+ \dfrac{1}{9}= \dfrac{5}{9}> \dfrac{1}{2}$
Vì $\dfrac{1}{10}+ \dfrac{1}{11}...+ \dfrac{1}{19}> \dfrac{1}{19}+ \dfrac{1}{19}+...+ \dfrac{1}{19}= \dfrac{10}{19}> \dfrac{1}{2}$
$B> \dfrac{1}{4}+ ( \dfrac{1}{5}+ \dfrac{10}{19}> \dfrac{1}{4}+ \dfrac{1}{2}+ \dfrac{1}{2}> 1$
$B> 1$
2. Đề bài
M= 3. (1/5+ 1/7- 1/11)/4. (1/5+ 1/7- 1/11)
$M= \dfrac{3}{4}$