\(A=\frac{1}{3.4}+\frac{1}{4.5}+....+\frac{1}{9.10}=\frac{1}{3}-\frac{1}{4}+.....+\frac{1}{9}-\frac{1}{10}\)
\(=\frac{1}{3}-\frac{1}{4}+.....+\frac{1}{9}-\frac{1}{10}\)
\(=\frac{1}{3}-\frac{1}{10}\)
\(=\frac{7}{30}\)
\(A=\frac{1}{3.4}+\frac{1}{4.5}+\frac{1}{5.6}+\frac{1}{6.7}+\frac{1}{7.8}+\frac{1}{8.9}+\frac{1}{9.10}\)
\(A=\frac{1}{3}-\frac{1}{4}+\frac{1}{4}-\frac{1}{5}+\frac{1}{5}-\frac{1}{6}+\frac{1}{6}-\frac{1}{7}+\frac{1}{7}-\frac{1}{8}+\frac{1}{8}-\frac{1}{9}+\frac{1}{9}-\frac{1}{10}\)
\(A=\frac{1}{3}-\frac{1}{10}\)
\(A=\frac{10}{30}-\frac{3}{30}=\frac{7}{30}\)
Vậy \(A=\frac{7}{30}\)
A, \(\frac{1}{3.4}\) + \(\frac{1}{4.5}\) + ...+ \(\frac{1}{9.10}\)
= \(\frac{1}{3}\) - \(\frac{1}{4}\) + ...+ \(\frac{1}{9}\) - \(\frac{1}{10}\)
= \(\frac{1}{3}\) - \(\frac{1}{10}\)
= \(\frac{7}{10}\)
sorry nha kết quả phải là 7/ 30 nhưng minh vít lộn thành 7/10 rùi
