\(A=\frac{1}{3}+\frac{1}{6}+...+\frac{1}{28}\)
\(A=\frac{2}{6}+\frac{2}{12}+...+\frac{2}{56}\)
\(A=\frac{2}{2.3}+\frac{2}{3.4}+...+\frac{2}{7.8}\)
\(A=2\left(\frac{1}{2}-\frac{1}{3}+\frac{1}{3}-\frac{1}{4}+...+\frac{1}{7}-\frac{1}{8}\right)\)
\(A=2\left(\frac{1}{2}-\frac{1}{8}\right)\)
\(A=2\frac{3}{8}=\frac{3}{4}\)
Ủng hộ mk nha !!! ^_^
\(A=\frac{1}{3}+\frac{1}{6}+\frac{1}{10}+\frac{1}{15}+\frac{1}{21}+\frac{1}{28}\)
\(A=\frac{2}{6}+\frac{2}{12}+\frac{2}{20}+\frac{2}{30}+\frac{2}{42}+\frac{2}{56}\) (Tử số và mẫu số mỗi phân số nhân với 2 thì giá trị ko thay đổi)
\(A=\frac{2}{2.3}+\frac{2}{3.4}+\frac{2}{4.5}+\frac{2}{5.6}+\frac{2}{6.7}+\frac{2}{7.8}\)
\(A:2=\frac{1}{2.3}+\frac{1}{3.4}+\frac{1}{4.5}+\frac{1}{5.6}+\frac{1}{6.7}+\frac{1}{7.8}\)
\(A:2=\frac{1}{2}-\frac{1}{3}+\frac{1}{3}-\frac{1}{4}+\frac{1}{4}-\frac{1}{5}+\frac{1}{5}-\frac{1}{6}+\frac{1}{6}-\frac{1}{7}+\frac{1}{7}-\frac{1}{8}\)
\(A:2=\frac{1}{2}-\frac{1}{8}=\frac{3}{8}\)
\(A=\frac{3}{8}.2=\frac{3}{4}\)
