Question

[Linh]

A = 1/20 +1/30 +1/42 +1/56 +1/72 + 1/90

Answer 1

[Dương Đức Hiệp]

\(A=\frac{1}{20}+\frac{1}{30}+\frac{1}{42}+\frac{1}{56}+\frac{1}{72}+\frac{1}{90}\)

\(A=\frac{1}{4\cdot5}+\frac{1}{5\cdot6}+\frac{1}{6\cdot7}+\frac{1}{7\cdot8}+\frac{1}{8\cdot9}+\frac{1}{9\cdot10}\)

\(A=\frac{1}{4}-\frac{1}{5}+\frac{1}{5}-...+\frac{1}{9}-\frac{1}{10}\)

\(A=\frac{1}{4}-\frac{1}{10}\)

\(A=\frac{3}{20}\)
 

Answer 2

[Sherlockichi Kudoyle]

\(A=\frac{1}{4.5}+\frac{1}{5.6}+...+\frac{1}{9.10}\)

\(A=\frac{1}{4}-\frac{1}{5}+\frac{1}{5}-\frac{1}{6}+...+\frac{1}{9}-\frac{1}{10}\)

\(A=\frac{1}{4}-\frac{1}{10}=\frac{5}{20}-\frac{2}{20}=\frac{3}{20}\)