\(\frac{1}{3.5}+\frac{1}{5.7}+...+\frac{1}{19.21}\)
\(=\frac{1}{2}\left(\frac{2}{3.5}+\frac{2}{5.7}+...+\frac{2}{19.21}\right)\)
\(=\frac{1}{2}\left(\frac{1}{3}-\frac{1}{5}+\frac{1}{5}-\frac{1}{7}+...+\frac{1}{19}-\frac{1}{21}\right)\)
\(=\frac{1}{2}\left(\frac{1}{3}-\frac{1}{21}\right)=\frac{1}{2}.\frac{7-1}{21}=\frac{1}{7}\)
\(\frac{1}{3\times5}+\frac{1}{5\times7}+...+\frac{1}{19\times21}\)
\(=\frac{1}{2}\times\left(\frac{2}{3\times5}+\frac{2}{5\times7}+...+\frac{2}{19\times21}\right)\)
\(=\frac{1}{2}\times\left(\frac{1}{3}-\frac{1}{5}+\frac{1}{5}-\frac{1}{7}+..+\frac{1}{19}-\frac{1}{21}\right)\)
\(=\frac{1}{2}\times\left(\frac{1}{3}-\frac{1}{21}\right)\)
\(=\frac{1}{2}\times\frac{2}{7}\)
\(=\frac{1}{7}\)
\(\frac{1}{3.5}+\frac{1}{5.7}+...+\frac{1}{19.21}\)
\(=\frac{1}{2}.\left(\frac{2}{3.5}+\frac{2}{5.7}+...+\frac{2}{19.21}\right)\)
\(=\frac{1}{2}.\left(\frac{1}{3}-\frac{1}{5}+\frac{1}{5}-\frac{1}{7}+...+\frac{1}{19}+\frac{1}{21}\right)\)
\(=\frac{1}{2}.\left(\frac{1}{3}-\frac{1}{21}\right)=\frac{1}{2}.\frac{6}{21}=\frac{1}{7}\)
