\(A=\frac{1}{2\cdot15}+\frac{3}{11\cdot2}+\frac{4}{1\cdot11}+\frac{5}{2\cdot1}\)

\(\Rightarrow\frac{1}{7}A=\frac{1}{7\cdot2\cdot15}+\frac{3}{11\cdot2\cdot7}+\frac{4}{7\cdot1\cdot11}+\frac{5}{2\cdot7\cdot1}\)

\(\Rightarrow\frac{1}{7}A=\frac{1}{15\cdot14}+\frac{3}{14\cdot11}+\frac{4}{11\cdot7}+\frac{5}{7\cdot2}\)

\(\Rightarrow\frac{1}{7}A=\frac{1}{14}-\frac{1}{15}+\frac{1}{14}-\frac{1}{11}+\frac{1}{11}-\frac{1}{7}+\frac{1}{7}-\frac{1}{5}\)

\(\Rightarrow\frac{1}{7}A=\frac{1}{14}-\frac{1}{5}=\frac{5}{70}-\frac{14}{70}=\frac{-9}{70}\Rightarrow A=\frac{-9}{70}:\frac{1}{7}=\frac{-9\cdot7}{1\cdot70}\)\(=\frac{-9\cdot7}{10\cdot7}=\frac{-9}{10}\)

cho bieu thuc: P=(1/1.2+1/3.4+1/5.6+...+1/59.60).31.32.33.....59.60 Chung minh rang P chia het cho 91

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

B/7 =  5/2.7 + 4/7.11 + 3/11.14 - 1/14.15 + 13/15.4

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

B/7 = 1/2 - 1/28

B/7 = 13/28

B = 13/18 . 7 = 13/4

K MK NA <3

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

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

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

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

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

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

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

B=5/1.2+4/1.11+3/11.2+1/2.15+13/15.4

B/7=5/2.7+4/7.11+3/11.14+1/14.15+13/15.4

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

B/7=1/2-1/28

B/7=13/28

B=13/28.7=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

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

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

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

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

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

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

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

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

    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