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

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

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

\(=\frac{1}{2}\left(\frac{1}{2}-\frac{1}{28}\right)\)

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

\(=\frac{13}{56}\)

Ta có

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

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

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

\(\Rightarrow\frac{1}{7}B=\frac{13}{28}\)

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

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

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

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

B:7=\(\frac{1}{2}-\frac{1}{28}\)=\(\frac{13}{28}\)=>B=\(\frac{13}{4}\)

mình không biết đâu nhé

mình mới học lớp 4 thôi

đáp số mình học lớp 4

bn viết như phân số đi mk thấy bn viết như kia là ko đúng, viết như phân số xong mk giải cho

nói luôn là không bt giải đi bày đặt

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

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

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

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

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

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

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

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

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

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

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

\(\frac{1}{7}C=\frac{13}{28}\)

\(C=\frac{13}{28}:\frac{1}{7}\)

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

Vậy \(C=\frac{13}{4}\)

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

-> 5/2+4/11+3/22+1/30+13/60

-> 1650/660+240/660+90/660+22/660+143/660

-> 2145/660

-> 13/4

5/2.1+4/1.11+3/11.2+1/12.15+13/15.4

=7.(5/2.7+4/7.11+3/11.14+1/14.15+13/15.28)

=7.(1/2-1/7+1/7-1/11+1/11-1/14+1/14-1/15+1/15-1/28)

=7.(1/2-1/28)=13/14=hỗn số 3 1 phần 4

\(B=\frac{5.7}{2.7}+\frac{4.7}{7.11}+\frac{3.7}{11.14}+\frac{1.7}{14.15}+\frac{13.7}{15.28}\)

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

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

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

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

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

uề

tiến y đan nhe 

hấn gửi câu hỏi là trả lời liền

còn choa gửi thì éo trả lời

\(n=7\left(\frac{5}{2.7}+\frac{4}{7.11}+\frac{3}{11.14}+\frac{1}{14.15}+\frac{13}{15.28}\right)\)

\(tacocongthuc:\frac{1}{n}-\frac{1}{n+k}=\frac{k}{n\left(n+k\right)}\)

\(\Rightarrow n=7\left(\frac{1}{2}-\frac{1}{7}+\frac{1}{7}-.....-\frac{1}{28}\right)=7\left(\frac{1}{2}-\frac{1}{28}\right)=\frac{7.13}{28}=\frac{13}{4}\)

#)Giải :

( k chép lại đề )

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

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

\(\frac{n}{7}=\frac{1}{2}-\frac{1}{28}=\frac{13}{28}\)

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