Question

[Đỗ Mỹ Y Hoa]

\(C=\frac{2}{1\cdot5}+\frac{2}{5\cdot9}+\frac{2}{9\cdot13}+\frac{2}{13\cdot17}+\frac{2}{17\cdot21}\)

Answer 1

[Nguyễn Hưng Phát]

\(C=\frac{2}{4}.\left(\frac{4}{1.5}+\frac{4}{5.9}+\frac{4}{9.13}+\frac{4}{13.17}+\frac{4}{17.21}\right)\)

\(=\frac{1}{2}.\left(1-\frac{1}{5}+\frac{1}{5}-\frac{1}{9}+....+\frac{1}{17}-\frac{1}{21}\right)\)

\(=\frac{1}{2}.\left(1-\frac{1}{21}\right)\)

\(=\frac{1}{2}.\frac{20}{21}\)

\(=\frac{10}{21}\)