Question

[tang bao nhi]

B=1/5+1/30+1/42+1/56+1/72+1/90

Answer 1

[Trần Đức Thắng]

B = 1/5 + 1/5.6 + 1/6.7 + 1/7.8 + 1/8.9 + 1/9.10

B = 1/5 + 1/5 - 1/6 + 1/6 - 1/7 + 1/7 - 1/8 + 1/8 - 1/9 + 1/9 - 1/10

B = 1/5 + 1/5 - 1/10

B = 2/5 - 1/10

B = 3/10

Answer 2

[Đinh Tuấn Việt]

\(B=\frac{1}{5}+\frac{1}{5.6}+\frac{1}{6.7}+...+\frac{1}{9.10}\)

    \(=\frac{1}{5}+\frac{1}{5}-\frac{1}{6}+\frac{1}{6}-\frac{1}{7}+...+\frac{1}{9}-\frac{1}{10}\)

    \(=\frac{1}{5}+\frac{1}{5}-\frac{1}{10}=\frac{4}{10}-\frac{1}{10}=\frac{3}{10}\)