\(B=\frac{7}{10.11}+\frac{7}{11.12}+...+\frac{7}{69.70}\)
\(B=7.\left(\frac{1}{10.11}+\frac{1}{11.12}+...+\frac{1}{69.70}\right)\)
\(B=7.\left(\frac{1}{10}-\frac{1}{11}+\frac{1}{11}-\frac{1}{12}+...+\frac{1}{69}-\frac{1}{70}\right)\)
\(B=7.\left(\frac{1}{10}-\frac{1}{70}\right)\)
\(B=7.\left(\frac{7}{70}-\frac{1}{70}\right)\)
\(B=7.\frac{6}{70}\)
\(B=\frac{3}{5}\)
Chúc bạn học tốt !!!