\(M=\dfrac{1}{1.3}+\dfrac{1}{3.5}+\dfrac{1}{5.7}+...+\dfrac{1}{995.997}+\dfrac{1}{997.999}\)
\(2M=\dfrac{2}{1.3}+\dfrac{2}{3.5}+\dfrac{2}{5.7}+...+\dfrac{2}{995.997}+\dfrac{2}{997.999}\)
\(=1-\dfrac{1}{3}+\dfrac{1}{3}-\dfrac{1}{5}+\dfrac{1}{5}-\dfrac{1}{7}+...+\dfrac{1}{997}-\dfrac{1}{999}\)
\(=1-\dfrac{1}{999}=\dfrac{998}{999}\)
\(\Rightarrow M=\dfrac{998}{999}.\dfrac{1}{2}=\dfrac{499}{999}\)
\(M=\dfrac{1}{1.3}+\dfrac{1}{3.5}+\dfrac{1}{5.7}+...+\dfrac{1}{995.997}+\dfrac{1}{997.999}\\ =\dfrac{1}{2}.\left(\dfrac{2}{1.3}+\dfrac{2}{3.5}+\dfrac{2}{5.7}+...+\dfrac{2}{995.997}+\dfrac{2}{997.999}\right)\\ =\dfrac{1}{2}.\left(1-\dfrac{1}{3}+\dfrac{1}{3}-\dfrac{1}{5}+\dfrac{1}{5}-\dfrac{1}{7}+...+\dfrac{1}{997}-\dfrac{1}{999}\right)\\ =\dfrac{1}{2}.\left(1-\dfrac{1}{999}\right)=\dfrac{1}{2}.\dfrac{998}{999}=\dfrac{499}{999}\)
\(M=\dfrac{1}{1.3}+\dfrac{1}{3.5}+\dfrac{1}{5.7}+...+\dfrac{1}{997.999}\)
Ta thấy:
\(\dfrac{1}{1.3}=1-\dfrac{1}{3};\dfrac{1}{3.5}=\dfrac{1}{3}-\dfrac{1}{5};\dfrac{1}{5.7}=\dfrac{1}{5}-\dfrac{1}{7};\)
\(...;\dfrac{1}{997.999}=\dfrac{1}{997}-\dfrac{1}{999}\)
\(\Rightarrow M=1-\dfrac{1}{3}+\dfrac{1}{3}-\dfrac{1}{5}+\dfrac{1}{5}-\dfrac{1}{7}+...+\)
\(\dfrac{1}{997}-\dfrac{1}{999}\)
\(\Rightarrow M=1-\dfrac{1}{999}=\dfrac{998}{999}\)
Vậy M=\(\dfrac{998}{999}\)
Help me !!! 
