Chứng minh rằng: B = 1/6 + 1/7 + 1/8 +...+ 1/18 + 1/19 < 2

* Hướng dẫn giải

$B=\left(\frac{1}{6}+\mathrm{...}\frac{1}{9}\right)+\left(\frac{1}{10}+\mathrm{...}+\frac{1}{19}\right)<\underset{4\text{s}o}{\underbrace{\frac{1}{4}+\mathrm{...}+\frac{1}{4}}}+\underset{10\text{s}o}{\underbrace{\frac{1}{10}+\mathrm{...}+\frac{1}{10}}}=2$