Cho S= 1/21+1/22+1/23+....+1/35. Chọn câu đúng

* Hướng dẫn giải

$S=\frac{1}{21}+\frac{1}{22}+\frac{1}{23}+\mathrm{...}+\frac{1}{35}$

$S=\left(\frac{1}{21}+\mathrm{...}+\frac{1}{25}\right)+\left(\frac{1}{26}+\mathrm{...}+\frac{1}{30}\right)+\left(\frac{1}{31}+\mathrm{...}+\frac{1}{35}\right)$

$S>\left(\frac{1}{25}+\mathrm{...}+\frac{1}{25}\right)+\left(\frac{1}{30}+\mathrm{...}+\frac{1}{30}\right)+\left(\frac{1}{35}+\mathrm{...}+\frac{1}{35}\right)$

$S>\frac{1}{5}+\frac{1}{6}+\frac{1}{7}=\frac{107}{210}>\frac{1}{2}$