Chứng minh rằng: A = 1/12 + 1/13 + ... + 1/22 > 1/2

* Hướng dẫn giải

$\text{A}=\frac{1}{12}+\frac{1}{13}+\mathrm{...}+\frac{1}{22}>\underset{11s}{\underbrace{\frac{1}{22}+\frac{1}{22}+\mathrm{...}+\frac{1}{22}}}=\frac{11}{22}=\frac{1}{2}$