Question

tính nhanh tổng A 

A=1/20+1/30+1/42+1/56+...+1/990

Answer 1

A=1/4.5+1/5.6+1/6.7+1/7.8+.....+1/...

hê,sai đề rùi

Answer 2

\(A=\frac{1}{20}+\frac{1}{30}+\frac{1}{42}+...+\frac{1}{990}\)

   \(=\frac{1}{4.5}+\frac{1}{5.6}+\frac{1}{6.7}+...+\frac{1}{30.31}\)

    \(=\frac{1}{4}-\frac{1}{5}+\frac{1}{5}-\frac{1}{6}+....+\frac{1}{30}-\frac{1}{31}\)

      \(=\frac{1}{4}-\frac{1}{31}\)

      \(=\frac{27}{124}\)