\(M=\frac{2}{30}+\frac{2}{42}+...+\frac{2}{1980}\)
\(M=2\left(\frac{1}{5.6}+\frac{1}{6.7}+...+\frac{1}{44.45}\right)\)
\(M=2\left(\frac{1}{5}-\frac{1}{6}+\frac{1}{6}-\frac{1}{7}+...+\frac{1}{44}-\frac{1}{45}\right)\)
\(M=2\left(\frac{1}{5}-\frac{1}{45}\right)\)
\(M=2\times\frac{8}{45}\)
\(M=\frac{16}{45}\)
\(M=\frac{1}{15}+\frac{1}{21}+\frac{1}{28}+....+\frac{1}{946}+\frac{1}{990}\)
\(M=\frac{2}{30}+\frac{2}{42}+\frac{2}{56}+.....+\frac{2}{1892}+\frac{2}{1980}\)
\(M=2.\left(\frac{1}{30}+\frac{1}{42}+\frac{1}{56}+...+\frac{1}{1892}+\frac{1}{1980}\right)\)
\(M=2.\left(\frac{1}{5.6}+\frac{1}{6.7}+\frac{1}{7.8}+....+\frac{1}{43.44}+\frac{1}{44.45}\right)\)
\(M=2.\left(\frac{1}{5}-\frac{1}{6}+\frac{1}{6}-\frac{1}{7}+\frac{1}{7}-\frac{1}{8}+....+\frac{1}{43}-\frac{1}{44}+\frac{1}{44}-\frac{1}{45}\right)\)
\(M=2.\left(\frac{1}{5}-\frac{1}{45}\right)=2.\frac{8}{45}=\frac{16}{45}\)
Vậy M=16/45
