Question

Cho A=\(\frac{4}{1.4}+\frac{4}{4.7}+\frac{4}{7.10}+..............+\frac{4}{67.70}\)

Chứng minh A>\(\frac{9}{7}\)

Answer 1

Có A=\(\frac{4}{1.4}+\frac{4}{4.7}+\frac{4}{7.10}+.........+\frac{4}{67.70}\)

      A=\(\frac{4}{3}.\left(\frac{3}{1.4}+\frac{3}{4.7}+............+\frac{3}{67.70}\right)\)

      A=\(\frac{4}{3}.\left(\frac{1}{1}-\frac{1}{4}+\frac{1}{4}-..........-\frac{1}{70}\right)\)

      A=\(\frac{4}{3}.\left(1-\frac{1}{70}\right)\)

      A=\(\frac{4}{3}.\frac{69}{70}=\frac{46}{35}\)

Vì \(\frac{46}{35}>\frac{9}{7}\) nên A>\(\frac{9}{7}\)

Answer 2

\(A=\frac{4}{3}.\left(\frac{1}{1}-\frac{1}{4}+\frac{1}{4}-....-\frac{1}{70}\right)\)

\(A=\frac{4}{3}.\left(1-\frac{1}{70}\right)=\frac{4}{3}\cdot\frac{69}{70}=\frac{46}{35}>\frac{9}{7}\)

Vậy A >9/7