\(\dfrac{1}{21}>\dfrac{1}{30}\)
\(\dfrac{1}{22}>\dfrac{1}{30}\)
...
\(\dfrac{1}{29}>\dfrac{1}{30}\)
\(\dfrac{1}{30}=\dfrac{1}{30}\)
Do đó: \(S=\dfrac{1}{21}+\dfrac{1}{22}+...+\dfrac{1}{30}>\dfrac{1}{30}+\dfrac{1}{30}+...+\dfrac{1}{30}=\dfrac{10}{30}=\dfrac{1}{3}\)
=>S>1/3