\(B=\dfrac{5}{2.1}+\dfrac{4}{1.11}+\dfrac{3}{11.2}+\dfrac{1}{2.15}+\dfrac{13}{15.4}\)

\(\dfrac{B}{7}=\dfrac{5}{2.7}+\dfrac{4}{7.11}+\dfrac{3}{11.14}+\dfrac{1}{14.15}+\dfrac{13}{15.4}\)

\(\dfrac{B}{7}=\dfrac{1}{2}-\dfrac{1}{7}+\dfrac{1}{7}-\dfrac{7}{11}+...+\dfrac{1}{15}-\dfrac{1}{28}\)

\(\dfrac{B}{7}=\dfrac{1}{2}-\dfrac{1}{28}\)

\(\dfrac{B}{7}=\dfrac{13}{28}\)

\(B=\dfrac{13}{28}.7=\dfrac{13}{4}\)

\(\frac{13}{4}\)

\(B=\frac{5}{2.1}+\frac{4}{1.11}+\frac{3}{11.2}+\frac{1}{2.15}+\frac{13}{15.4}\)

\(\Rightarrow\frac{B}{7}=\frac{5}{2.7}+\frac{4}{7.11}+\frac{3}{11.14}+\frac{1}{14.15}+\frac{13}{15.28}\)

\(\Rightarrow\frac{B}{7}=\frac{1}{2}-\frac{1}{7}+\frac{1}{7}-\frac{1}{11}+\frac{1}{11}-\frac{1}{14}+\frac{1}{14}-\frac{1}{15}+\frac{1}{15}-\frac{1}{28}\)

\(\Rightarrow\frac{B}{7}=\frac{1}{2}-\frac{1}{28}\)

\(\Rightarrow\frac{B}{7}=\frac{14}{28}-\frac{1}{28}\)

\(\Rightarrow\frac{B}{7}=\frac{13}{28}\)

\(\Rightarrow B=\frac{13}{28}.7\)

\(\Rightarrow B=\frac{13}{4}\)

B=\(\frac{13}{4}\)

\(\frac{13}{4}\)

\(B=\frac{5}{2.1}+\frac{4}{1.11}+\frac{3}{11.2}+\frac{2}{2.15}+\frac{13}{15.4}\)

\(B=\frac{1}{2}-1+1-\frac{1}{11}+\frac{1}{11}-\frac{1}{2}+\frac{1}{2}-\frac{1}{15}+\frac{1}{15}-\frac{1}{4}\)

\(B=\frac{1}{2}-\frac{1}{4}\)

\(B=\frac{1}{4}\)

đáp án là:1/4
 

=13/4

Gọi tổng trên là A

A = 7.(5/2.7 + 4/7.11 + 3/11.14 + 1/14.15 + 1/15.28)

A = 7.(1/2 - 1/7 + 1/7 - 1/11 +........+1/15 - 1/28)

A = 7.(1/2 - 1/28)

A = 7. 13/28

A = 13/4

Vậy A = 13/4 nhé!

\(\Rightarrow\frac{B}{7}=\frac{5}{2.7}+\frac{4}{7.11}+\frac{3}{11.14}+\frac{1}{14.15}+\frac{13}{15.28}\)

\(=\frac{1}{2}-\frac{1}{7}+\frac{1}{7}-\frac{1}{11}+\frac{1}{11}-\frac{1}{14}+\frac{1}{14}-\frac{1}{15}+\frac{1}{15}-\frac{1}{28}\)

\(=\frac{1}{2}-\frac{1}{28}=\frac{13}{28}\)

\(\Rightarrow B=\frac{13}{28}.7=\frac{13}{4}\)

13/4 nha bạn

     B=5/2.1+4/1.11+3/11.2+1/2.15+13/15.4

=>1/7.B= 5/2.7+4/7.11+3/11.14+1/14.15+13/15.28

=>1/7.B=1/2-1/7+1/7-1/11+1/11-1/14+1/14-1/15+1/15-1/28

=>1/7.B=1/2-1/28

=>1/7.B=13/28

=>B= 13/28:1/7=13/4

 Vậy B=13/4