Ta có các p/s: $\frac{1}{11}$ , $\frac{1}{12}$, $\frac{1}{13}$, $\frac{1}{14}$, $\frac{1}{15}$, $\frac{1}{16}$, $\frac{1}{17}$, $\frac{1}{18}$, $\frac{1}{19}$ đều lớn hơn $\frac{1}{20}$.
Do đó : $\frac{1}{11}$, $\frac{1}{12}$, $\frac{1}{13}$, $\frac{1}{14}$, $\frac{1}{15}$, $\frac{1}{16}$, $\frac{1}{17}$, $\frac{1}{18}$, $\frac{1}{19}$, $\frac{1}{20}$ > $\frac{1}{20}$ + $\frac{1}{20}$+ $\frac{1}{20}$ + ...+$\frac{1}{20}$ ( có 10 p/s $\frac{1}{20}$)
$\frac{1}{11}$, $\frac{1}{12}$, $\frac{1}{13}$, $\frac{1}{14}$, $\frac{1}{15}$, $\frac{1}{16}$, $\frac{1}{17}$, $\frac{1}{18}$, $\frac{1}{19}$, $\frac{1}{20}$ > $\frac{10}{20}$.
$\frac{1}{11}$, $\frac{1}{12}$, $\frac{1}{13}$, $\frac{1}{14}$, $\frac{1}{15}$, $\frac{1}{16}$, $\frac{1}{17}$, $\frac{1}{18}$, $\frac{1}{19}$, $\frac{1}{20}$ > $\frac{1}{2}$.
Vậy S> $\frac{1}{2}$.
Chúc bn hc tốt nha ʕ•ﻌ•ʔʕ•ﻌ•ʔ
