Question

D = 1/2.5 + 1/5.8 + 1/8.11 +.... + 1/1979.1982.

NhỜ MọI NgƯờI GiÚp NhÉ.

Answer 1

\(D=\frac{1}{2.5}+\frac{1}{5.8}+\frac{1}{8.11}+...+\frac{1}{1979.1982} \)

    \(=\frac{1}{3}\left(\frac{3}{2.5}+\frac{3}{5.8}+\frac{3}{8.11}+...+\frac{3}{1979.1982}\right)\)

    \(=\frac{1}{3}\left(\frac{1}{2}-\frac{1}{5}+\frac{1}{5}-\frac{1}{8}+\frac{1}{8}-\frac{1}{11}+...+\frac{1}{1979}-\frac{1}{1982}\right)\)

    \(=\frac{1}{3}\left(\frac{1}{2}-\frac{1}{1982}\right)\)

    \(=\frac{165}{991}\)