đặt A=\(\frac{5}{2.1}+\frac{4}{1.11}+\frac{3}{11.2}+\frac{1}{2.15}+\frac{13}{15.4}\)
\(\frac{1}{7}A=\frac{5}{2.7.1}+\frac{4}{7.1.11}+\frac{3}{11.2.7}+\frac{1}{2.15.7}+\frac{13}{15.4.7}\)
\(\frac{1}{7}A=\frac{5}{7.2}+\frac{4}{7.11}+\frac{3}{11.14}+\frac{1}{14.15}+\frac{13}{15.28}\)
\(\frac{1}{7}A=\frac{1}{2}-\frac{1}{7}+\frac{1}{7}-\frac{1}{11}+\frac{1}{11}-\frac{1}{14}+\frac{1}{14}-\frac{1}{15}+\frac{1}{15}-\frac{1}{28}\)
\(\frac{1}{7}A=\frac{1}{2}-\frac{1}{28}=\frac{13}{28}\Rightarrow A=\frac{13}{4}\)
p/s: đối vs mấy bn lớp 6 có lẽ bài này ko ez lắm :))