Question

Tính tổng C = 1/15 + 1/21 + 1/28 +...+ 1/190

Answer 1

\(C=\frac{1}{15}+\frac{1}{21}+\frac{1}{28}+...+\frac{1}{190}\)

\(C=\frac{2}{30}+\frac{2}{42}+\frac{2}{56}+...+\frac{2}{380}\)

\(C=2\left(\frac{1}{5.6}+\frac{1}{6.7}+\frac{1}{7.8}+...+\frac{1}{19.20}\right)\)

\(C=2\left(\frac{1}{5}-\frac{1}{6}+\frac{1}{6}-\frac{1}{7}+\frac{1}{7}-\frac{1}{8}+...+\frac{1}{19}-\frac{1}{20}\right)\)

\(C=2\left(\frac{1}{5}-\frac{1}{20}\right)\)

\(C=2\cdot\frac{3}{20}=\frac{3}{10}\)