\(C=\frac{1}{30}+\frac{1}{42}+\frac{1}{56}+\frac{1}{72}\)
\(C=\frac{1}{5\cdot6}+\frac{1}{6\cdot7}+\frac{1}{7\cdot8}+\frac{1}{8\cdot9}\)
\(C=\frac{1}{5}-\frac{1}{6}+\frac{1}{6}-\frac{1}{7}+\frac{1}{7}-\frac{1}{8}+\frac{1}{8}-\frac{1}{9}\)
\(C=\frac{1}{5}-\frac{1}{9}=\frac{4}{45}\)
\(D=\frac{1}{1\cdot4}+\frac{1}{4\cdot7}+\frac{1}{7\cdot10}+\frac{1}{10\cdot13}+\frac{1}{13\cdot16}\)
\(D=\frac{1}{3}\cdot\left(\frac{1}{1}-\frac{1}{4}+\frac{1}{4}-\frac{1}{7}+\frac{1}{7}-\frac{1}{10}+\frac{1}{10}-\frac{1}{13}+\frac{1}{13}-\frac{1}{16}\right)\)
\(D=\frac{1}{3}\cdot\left(\frac{1}{1}-\frac{1}{16}\right)\)
\(D=\frac{1}{3}\cdot\frac{15}{16}=\frac{5}{16}\)
\(C=\frac{1}{30}+\frac{1}{42}+\frac{1}{56}+\frac{1}{72}\)
\(C=\frac{1}{5.6}+\frac{1}{6.7}+\frac{1}{7.8}+\frac{1}{8.9}\)
\(C=\frac{1}{5}-\frac{1}{6}+\frac{1}{6}-\frac{1}{7}+\frac{1}{7}-\frac{1}{8}+\frac{1}{8}-\frac{1}{9}\)
\(C=\frac{1}{5}-\frac{1}{9}\)
\(C=\frac{4}{45}\)
\(D=\frac{1}{1.4}+\frac{1}{4.7}+\frac{1}{7.10}+\frac{1}{10.13}+\frac{1}{13.16}\)
\(D=\frac{1}{3}\left(1-\frac{1}{4}+\frac{1}{4}-\frac{1}{7}+\frac{1}{7}-\frac{1}{10}+...+\frac{1}{13}-\frac{1}{16}\right)\)
\(D=\frac{1}{3}\left(1-\frac{1}{16}\right)\)
\(D=\frac{1}{3}.\frac{15}{16}\)
\(D=\frac{5}{16}\)
