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\(A=\left(\frac{1}{11}+\frac{1}{12}+...+\frac{1}{20}\right)+\left(\frac{1}{21}+...+\frac{1}{30}\right)+\left(\frac{1}{31}+...+\frac{1}{40}\right)+\left(\frac{1}{41}+...+\frac{1}{50}\right)+\left(\frac{1}{51}+...+\frac{1}{60}\right)+\left(\frac{1}{61}+...+\frac{1}{70}\right)\)nhận xét

\(\frac{1}{11}+\frac{1}{12}+...+\frac{1}{20}<\frac{1}{11}+...+\frac{1}{11}=\frac{10}{11}<\frac{10}{10}=1\)

\(\frac{1}{21}+\frac{1}{22}+...+\frac{1}{30}<\frac{1}{21}+...+\frac{1}{21}=\frac{10}{21}<\frac{10}{20}=\frac{1}{2}\)

\(\frac{1}{31}+\frac{1}{32}+...+\frac{1}{40}<\frac{1}{31}+...+\frac{1}{40}=\frac{10}{31}<\frac{10}{30}=\frac{1}{3}\)

\(\frac{1}{41}+\frac{1}{42}+...+\frac{1}{50}<\frac{1}{41}+...+\frac{1}{41}=\frac{10}{41}<\frac{10}{40}=\frac{1}{4}\)

\(\frac{1}{51}+\frac{1}{52}+...+\frac{1}{60}<\frac{1}{51}+...+\frac{1}{60}=\frac{10}{51}<\frac{10}{50}=\frac{1}{5}\)

\(\frac{1}{61}+\frac{1}{62}+...+\frac{1}{70}<\frac{1}{61}+...+\frac{1}{61}=\frac{10}{61}<\frac{10}{60}=\frac{1}{6}\)

\(A<1+\frac{1}{2}+\frac{1}{3}+\frac{1}{4}+\frac{1}{5}+\frac{1}{6}=1+\left(\frac{1}{2}+\frac{1}{3}+\frac{1}{6}\right)+\left(\frac{1}{4}+\frac{1}{5}\right)<1+1+\frac{1}{2}=\frac{5}{2}\)