Chứng minh rằng: A = 1/12 + 1/13 + 1/14 +... + 1/22 >1/2

* Hướng dẫn giải

$A=\frac{1}{12}+\frac{1}{13}+\frac{1}{14}+\mathrm{...}+\frac{1}{22}>\underset{11\text{s}}{\underbrace{\frac{1}{22}+\frac{1}{22}+\mathrm{...}\frac{1}{22}}}=\frac{11}{22}=\frac{1}{2}.$