Question

[Nguyễn Trà My]

Tính giá trị biểu thức:

A =1/30 + 1/42 + 1/56 + 1/72 + 1/90 +1/110 + 1/132

Answer 1

[Nguyễn Ngọc Anh Minh]

\(A=\frac{1}{5.6}+\frac{1}{6.7}+\frac{1}{7.8}+\frac{1}{8.9}+\frac{1}{9.10}+\frac{1}{10.11}+\frac{1}{11.12}=\frac{6-5}{5.6}+\frac{7-6}{6.7}+...+\frac{12-11}{11.12}\)

\(A=\frac{1}{5}-\frac{1}{6}+\frac{1}{6}-\frac{1}{7}+...+\frac{1}{11}-\frac{1}{12}=\frac{1}{5}-\frac{1}{12}=\frac{7}{60}\)