Sửa đề :\(\frac{5}{11.12}+\frac{5}{12.13}+\frac{5}{13.14}+\frac{5}{14.15}\)
\(=5\left(\frac{1}{11}-\frac{1}{12}+\frac{1}{12}-\frac{1}{13}+\frac{1}{13}-\frac{1}{14}+\frac{1}{14}-\frac{1}{15}\right)\)
\(=5\left(\frac{1}{11}-\frac{1}{15}\right)\)
\(=\frac{5}{11}-\frac{1}{3}\)
\(=\frac{15-11}{33}=\frac{14}{33}\)
\(A = {5\over10}-{5\over11}+{5\over11}-{5\over12}+...+{5\over14}-{5\over15} = {5\over10}-{5\over15}={1\over6} \)Ban chep de thieu nha^^
\(\frac{5}{10.11}+\frac{5}{12.13}+\frac{5}{13.14}+\frac{5}{14.15}\)
\(=5.\left(\frac{1}{10}-\frac{1}{11}+\frac{1}{11}-\frac{1}{12}+\frac{1}{12}-\frac{1}{13}+\frac{1}{13}-\frac{1}{14}+\frac{1}{14}-\frac{1}{15}\right)\)
\(=5\left(\frac{1}{10}-\frac{1}{15}\right)\)
\(=5.\frac{5}{150}\)
\(=\frac{25}{150}=\frac{1}{6}\)
