\(A=\frac{2}{2.5}+\frac{2}{5.8}+\frac{2}{8.11}+...+\frac{2}{17.20}+\frac{2}{20.23}\)
\(\frac{3A}{2}=\frac{3}{2.5}+\frac{3}{5.8}+\frac{3}{8.11}+...+\frac{3}{17.20}+\frac{3}{20.23}\)
\(\frac{3A}{2}=\frac{5-2}{2.5}+\frac{8-5}{5.8}+\frac{11-8}{8.11}+...+\frac{20-17}{17.20}+\frac{23-20}{20.23}\)
\(\frac{3A}{2}=\frac{1}{2}-\frac{1}{5}+\frac{1}{5}-\frac{1}{8}+\frac{1}{8}-\frac{1}{11}+...+\frac{1}{17}-\frac{1}{20}+\frac{1}{20}-\frac{1}{23}=\frac{1}{2}-\frac{1}{23}=\frac{21}{46}\)
\(A=\frac{21.2}{46.3}=\frac{7}{23}\)
Mình trả lời trong tin nhắn rồi xem có đúng không nhé
