Question

Tính giá trị biểu thức

7²/2×9 + 7²/9×16 + ... + 7²/65×72

Answer 1

\(\frac{7^2}{2.9}+\frac{7^2}{9.16}+...+\frac{7^2}{65.72}\)

\(=7^2\left(\frac{1}{2.9}+\frac{1}{9.16}+...+\frac{1}{65.72}\right)\)

\(=\frac{7^2}{7}\left(\frac{1}{2}-\frac{1}{9}+\frac{1}{9}-\frac{1}{16}+...+\frac{1}{65}-\frac{1}{72}\right)\)

\(=7\left(\frac{1}{2}-\frac{1}{72}\right)\)

\(=7.\frac{35}{72}\)

\(=\frac{245}{72}\)