đáp số
\(\frac{13}{4}\)
hok tốt
\(B=\frac{5}{2.1}+\frac{4}{1.11}+\frac{3}{11.2}+\frac{1}{2.15}+\frac{13}{15.4}\)
\(=\frac{5}{2}+\frac{1}{11}\left(\frac{4}{1}+\frac{3}{2}\right)+\frac{1}{15}\left(\frac{1}{2}+\frac{13}{4}\right)\)
\(=\frac{5}{2}+\frac{1}{11}\left(\frac{8+3}{2}\right)+\frac{1}{15}\left(\frac{2+13}{4}\right)\)
\(=\frac{5}{2}+\frac{1}{11}.\frac{11}{2}+\frac{1}{15}.\frac{15}{4}\)
\(=\frac{5}{2}+\frac{1}{2}+\frac{1}{4}\)
\(=3+\frac{1}{4}=3\frac{1}{4}\)
\(B=\frac{5}{2.1}+\frac{4}{1.11}+\frac{3}{11.2}+\frac{1}{2.15}+\frac{13}{15.4}\)
\(\Rightarrow\frac{B}{7}=\frac{5}{2.7}+\frac{4}{7.11}+\frac{3}{11.14}+\frac{1}{14.15}+\frac{13}{15.28}\)
\(\Rightarrow\frac{B}{7}=\frac{1}{2}-\frac{1}{7}+\frac{1}{7}-\frac{1}{11}+\frac{1}{11}-\frac{1}{14}+\frac{1}{14}-\frac{1}{15}+\frac{1}{15}-\frac{1}{28}\)
\(\Rightarrow\frac{B}{7}=\frac{1}{2}-\frac{1}{28}\)
\(\Rightarrow\frac{B}{7}=\frac{14}{28}-\frac{1}{28}\)
\(\Rightarrow\frac{B}{7}=\frac{13}{28}\)
\(\Rightarrow B=\frac{13}{28}.7\)
\(\Rightarrow B=\frac{13}{4}\)
Chúc bạn học tốt !!!
