\(\frac{4}{3.7}+\frac{5}{7.12}+\frac{1}{12.13}+\frac{7}{13.20}+\frac{3}{20.23}\)
\(=\frac{1}{3}-\frac{1}{7}+\frac{1}{7}-\frac{1}{12}+...+\frac{1}{20}-\frac{1}{23}\)
\(=\frac{1}{3}-\frac{1}{23}+0+0+0\)
\(=\frac{20}{69}\)
Dấu "." là dấu nhân nha
Đặt \(A=\frac{4}{3\cdot7}+\frac{5}{7\cdot12}+\frac{1}{12\cdot13}+\frac{7}{13\cdot20}+\frac{3}{20\cdot23}\)
\(A=\frac{7-3}{3\cdot7}+\frac{12-7}{7\cdot12}+\frac{13-12}{12\cdot13}+\frac{20-13}{13\cdot20}+\frac{23-20}{20\cdot23}\)
\(A=\frac{1}{3}-\frac{1}{7}+....+\frac{1}{20}-\frac{1}{23}\)
\( A=\frac{1}{3}-\frac{1}{23}=\frac{20}{69}\)
