Sao ko ai giup vây HuHu

?????

=12/3+12/11+12/15+..+12/43+12/47

=4+12/11+3/5+..+12/43+12/47

Ta có :

\(\frac{4}{3.7}+\frac{4}{7.11}+\frac{4}{11.15}+....+\frac{4}{23.27}\)

\(=\frac{1}{3}-\frac{1}{7}+\frac{1}{7}-\frac{1}{11}+\frac{1}{11}-\frac{1}{15}+....+\frac{1}{23}-\frac{1}{27}\)

\(=\frac{1}{3}-\frac{1}{27}==\frac{9}{27}-\frac{1}{27}=\frac{8}{27}\)

Đặt \(A=\frac{4}{3.7}+\frac{4}{7.11}+\frac{4}{11.15}+\frac{4}{15.19}++\frac{4}{19.23}+\frac{4}{23.27}\)

\(A=\frac{1}{3}-\frac{1}{7}+\frac{1}{7}-\frac{1}{11}+\frac{1}{11}-\frac{1}{15}+...+\frac{1}{23}-\frac{1}{27}\)

\(A=\frac{1}{3}-\frac{1}{27}\)

\(A=\frac{8}{27}\)

\(\frac{4}{3\cdot7}+\frac{4}{7\cdot11}+\frac{4}{11\cdot15}+\frac{4}{15\cdot19}+\frac{4}{19\cdot23}+\frac{4}{23.27}\)

\(=\frac{4}{4}\cdot\left(\frac{1}{3}-\frac{1}{7}+\frac{1}{7}-\frac{1}{11}+...+\frac{1}{23}-\frac{1}{27}\right)\)

\(=\frac{4}{4}\cdot\left(\frac{1}{3}-\frac{1}{27}\right)\)

\(=\frac{4}{4}\cdot\frac{8}{27}=\frac{8}{27}\)

\(\frac{1}{3}-\frac{1}{2027}\)

a, -5/12 + 1/-4 = -5/12 + -1/4 = -5/12 + -3/12 = -8/12 = -2/3

b, 5/12 + -3/28 = 35/84 + -9/84 = 26/84 = 13/42

c, -7/12 + 3/55 = -385/660 + 36/660 = -349/660 

d, -5/7 + -3/4 = -20/28 + -21/28 = -41/28

ai giải giúp mk, mk và kb, ib lun

mk ko mún làm

thanks bn nhìu!!!

Ta có \(81.\left(\frac{12-\frac{12}{7}-\frac{12}{289}-\frac{12}{85}}{4-\frac{4}{7}-\frac{4}{289}-\frac{3}{85}}:\frac{5+\frac{5}{13}+\frac{5}{169}+\frac{5}{91}}{6+\frac{6}{13}+\frac{6}{169}+\frac{6}{91}}\right)\)

\(=81.\left(\frac{12.\left(1-\frac{1}{7}-\frac{1}{289}-\frac{1}{85}\right)}{4.\left(1-\frac{1}{7}-\frac{1}{289}-\frac{1}{85}\right)}:\frac{5.\left(1+\frac{1}{13}+\frac{1}{169}+\frac{1}{91}\right)}{6.\left(1+\frac{1}{13}+\frac{1}{169}+\frac{1}{91}\right)}\right)\)

\(=81.\left(\frac{12}{4}:\frac{5}{6}\right)\)

\(=81.\frac{18}{5}\)

\(=291,6\)

\(81\left(\frac{12-\frac{12}{7}-\frac{12}{289}-\frac{12}{85}}{4-\frac{4}{7}-\frac{4}{289}-\frac{4}{85}}:\frac{5+\frac{5}{13}+\frac{5}{169}+\frac{5}{91}}{6+\frac{6}{13}+\frac{6}{159}+\frac{6}{91}}\right)\)

\(=81\left(\frac{12\left(1-\frac{1}{7}-\frac{1}{289}-\frac{1}{85}\right)}{4\left(1-\frac{1}{7}-\frac{1}{289}-\frac{1}{85}\right)}:\frac{5\left(1+\frac{1}{13}+\frac{1}{169}+\frac{1}{91}\right)}{6\left(1+\frac{1}{13}+\frac{2}{169}+\frac{1}{91}\right)}\right)\)

\(=81\left(3\div\frac{5}{6}\right)\)

\(=81.\frac{18}{5}\)

\(=\frac{1458}{5}\)

81.[12(1-1/7-1/289-1/85)/4(1-1/7-1/289-1/85):5(1+1/13+1/169+1/91)/6(1/13+1/169+1/91)

=81.[12/4:5/6]=81.[12/4.6/5]=81.18/5=1458/5

Đặt A = 9/4  + 9/28 +.. + 9/550

       A = 9/1.4 + 9/4.7 +... + 9/22.25

       A = 3( 3/1.4 + 3/4.7 + .. + 3/22.25)

        A  = 3 . (1/1 - 1/4 + 1/4  - 1/7 + ... +1/22 - 1/25)

         A = 3 (1 - 1/25)

       A   = 3. 24 / 25

       A   =  72/25

Gọi số cần tìm là \(x\), ta có :

S = \(\frac{4}{3x7}\)+  \(\frac{4}{7x11}\)+ \(\frac{4}{11x15}\)+ ............\(x\) = \(\frac{664}{1995}\)

 = \(\frac{4}{3}\)- \(\frac{4}{7}\)+ \(\frac{4}{7}\) -  \(\frac{4}{11}\)+  \(\frac{4}{11}\) -  \(\frac{4}{15}\)+ ..............\(x\) = \(\frac{664}{1995}\)

 = \(\frac{4}{3}\)-  \(x\)=  \(\frac{664}{1995}\)( loại các sô giống nhau )

\(x\)= \(\frac{4}{3}\)-  \(\frac{664}{1995}\)

\(x\)=  \(\frac{1996}{1995}\)

a.Goi so cuoi la x ta co

....................(de bai)

=1/3-1/7+1/7-1/11+1/11-1/15+...-x=664/1995

=1/3-x=664/1995

x=1/3-664/1995

x=1/1995

BN HOANG HIEU OI BN TINH HOI NHAM RUI!BN THU XEM LAI HIEU CUA MAU SO SO VOI TU SO DI!!!