Question

[Phạm Hà Hiểu Sương]

12/2.5 + 12/5.8 + 12/8.11 + ... + 12/65.68

Answer 1

[Phùng Minh Quân]

Ta có : 

\(\frac{12}{2.5}+\frac{12}{5.8}+\frac{12}{8.11}+...+\frac{12}{65.68}\)

\(=\)\(4\left(\frac{3}{2.5}+\frac{3}{5.8}+\frac{3}{8.11}+...+\frac{3}{65.68}\right)\)

\(=\)\(4\left(\frac{1}{2}-\frac{1}{5}+\frac{1}{5}-\frac{1}{8}+\frac{1}{8}-\frac{1}{11}+...+\frac{1}{65}-\frac{1}{68}\right)\)

\(=\)\(4\left(\frac{1}{2}-\frac{1}{68}\right)\)

\(=\)\(2-\frac{1}{17}\)

\(=\)\(\frac{35}{17}\)

Vậy \(\frac{12}{2.5}+\frac{12}{5.8}+\frac{12}{8.11}+...+\frac{12}{65.68}=\frac{35}{17}\)

Chúc bạn học tốt ~