Ta có: B = \(\frac{6}{15}+\frac{6}{35}+\frac{6}{63}+\frac{6}{99}\)
=> B = \(\frac{6}{3.5}\)+ \(\frac{6}{5.7}\)+ \(\frac{6}{7.9}\)+ \(\frac{6}{9.11}\)
=>B =\(3.\left(\frac{2}{3.5}+\frac{2}{5.7}+\frac{2}{7.9}+\frac{2}{9.11}\right)\)
=> B = \(3.\left(\frac{1}{3}-\frac{1}{5}+\frac{1}{5}-\frac{1}{7}+\frac{1}{7}-\frac{1}{9}+\frac{1}{9}-\frac{1}{11}\right)\)
=> B = \(3.\left(\frac{1}{3}-\frac{1}{11}\right)\)
=> B = \(3.\frac{8}{33}\)
=> B = \(\frac{8}{11}\)
Vậy: B = \(\frac{8}{11}\)