A=1/3.7+1/7.11+1/11.15+...+1/91.95

=>4A=4/3.7+4/7.11+4/11.15+...+4/91.95

    4A=1/3-1/7+1/7-1/11+1/11-1/15+...1/91-1/95

    4A=1/3-1/95

    4A=92/285

      A=92/285:4

      A=23/285

Thank

Bạn chỉ cần lấy 1/3 trừ 1/95 là đc r, ko cần : 4 đâu

^^!

E=\(\frac{5}{1.2}+\frac{4}{1.11}+\frac{3}{11.2}+\frac{1}{2.15}+\frac{13}{15.4}\)

E.\(\frac{1}{7}\)=\(\frac{5}{1.2.7}+\frac{4}{1.11.7}+\frac{3}{11.2.7}+\frac{1}{2.15.7}+\frac{13}{15.4.7}\)

E.\(\frac{1}{7}\)=\(\frac{5}{7.2}+\frac{4}{7.11}+\frac{3}{11.14}+\frac{1}{14.15}+\frac{13}{15.28}\)

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

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

E.\(\frac{1}{7}=\frac{13}{28}\)

E=\(\frac{13}{28}:\frac{1}{7}=\frac{13}{4}\)

\(E=\frac{5}{1.2}+\frac{1}{1.11}+\frac{3}{11.2}+\frac{1}{2.15}+\frac{13}{15.4}\)

\(E=\frac{5}{2}+\frac{1}{11}+\frac{3}{22}+\frac{1}{30}+\frac{13}{60}\)

\(E=\frac{5}{2}+\left(\frac{1}{11}+\frac{3}{22}\right)+\left(\frac{1}{30}+\frac{13}{60}\right)\)

\(E=\frac{5}{2}+\left(\frac{2}{22}+\frac{3}{22}\right)+\left(\frac{2}{60}+\frac{13}{60}\right)\)

\(E=\frac{5}{2}+\frac{5}{22}+\frac{15}{60}\)

\(E=\frac{55}{22}+\frac{5}{22}+\frac{1}{4}\)

\(E=\frac{60}{22}+\frac{1}{4}\)

\(E=\frac{30}{11}+\frac{1}{4}\)

\(E=\frac{120}{44}+\frac{11}{44}\)\(=\frac{131}{44}\)

k mình nha chúc bạn học giỏi

\(B=\frac{5}{2}+\left(\frac{4}{1.11}+\frac{3}{11.2}\right)+\left(\frac{1}{2.15}+\frac{13}{15.4}\right)\)

\(B=\frac{5}{2}+\frac{1}{11}.\left(4+\frac{3}{2}\right)+\frac{1}{15}\left(\frac{1}{2}+\frac{13}{4}\right)=\frac{5}{2}+\frac{1}{11}.\frac{11}{2}+\frac{1}{15}.\frac{15}{4}\)

=> \(B=\frac{5}{2}+\frac{1}{2}+\frac{1}{4}=\frac{10}{4}+\frac{2}{4}+\frac{1}{4}=\frac{13}{4}\)

Ta có

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 = 14/28 - 1/28

1/7.B = 13/28

B = 13/28 : 1/7

B = 13/28 . 7

B = 13/4

Ta có :

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

\(B=7\left(\frac{5}{2.1}+\frac{4}{1.11}+\frac{3}{11.2}+\frac{1}{2.15}+\frac{13}{15.4}\right)\)

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

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

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

      \(=7.\frac{13}{28}\)

      \(=\frac{13}{4}\)

đề sai rồi 13/15.4 chứ

B.1/7=1/7.(5/2.1+4/1.11+3/11.2+1/2.15+13/15.14)

B.1/7=5/2.1.7+4/1.11.7+3/11.2.7+1/2.15.7+13/15.4.7)

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

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

B.1/7=1/2-1/28=13/28

B=13/28:1/7

B=13/4

lộn đề rồi hay gì ấy!!

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

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 . ( 1/2 - 1/28 )

B = 7 . 13/28

B = 13/4

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

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

1/7B= 1/2 - 1/28= 13/28

B= 13/28 : 1/7= 13/28.7= 13/4

Vậy B= 13/4

    \(\frac{1}{2.15}+\frac{3}{2.11}+\frac{4}{1.11}+\frac{5}{1.2}\)

\(=\frac{1}{30}+\left(\frac{3}{22}+\frac{4}{11}\right)+\frac{5}{2}\)

\(=\frac{1}{30}+\frac{1}{2}+\frac{5}{2}\)

\(=\frac{1}{30}+3\)

\(=\frac{91}{30}\)

= 3 \(\frac{1}{30}\)Hoặc  =\(\frac{91}{30}\)

ta có 

1/15.2=1/30=11/330

3/2.11=3/22=45/330

4/1.11=4/11=120/330

5/1.2=5/2=825/330

=>11/330+45/330+120/330+825/330

=11+45+120+825/330

=1001/330=91/330

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

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

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

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

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

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