\(\Leftrightarrow x-\dfrac{1}{4}=\dfrac{20}{27}\)

hay x=20/27+1/4=107/108

\(x-\dfrac{1}{4}=\dfrac{5}{3}\times\dfrac{4}{9}\\ \Rightarrow x-\dfrac{1}{4}=\dfrac{20}{27}\\ \Rightarrow x=\dfrac{20}{27}+\dfrac{1}{4}\\ \Rightarrow x=\dfrac{107}{108}\)

=>x-1/4=20/27

hay x=107/108

cảm  ơn  bạn 

\(x-\dfrac{1}{4}=\dfrac{5}{6}\times\dfrac{4}{9}\)

<=>\(x-\dfrac{1}{4}=\dfrac{10}{27}\)

<=>\(x=\dfrac{10}{27}+\dfrac{1}{4}=\dfrac{67}{108}\)

\(x-\dfrac{1}{4}=\dfrac{5}{6}\times\dfrac{4}{9}\\ \Rightarrow x-\dfrac{1}{4}=\dfrac{10}{27}\\ \Rightarrow x=\dfrac{10}{27}+\dfrac{1}{4}\\ \Rightarrow x=\dfrac{67}{108}\)

7/48 - (1/2 x 2 + 1/6 x 4 + 1/8 x 5 + 1/12 x 7 + 1/14 x 8) : x = 0

7/48 - (1 + 2/3 + 5/8 + 7/12 + 4/7) : x = 0 (đã rút gọn)

7/48 - (336/336 + 224/336 + 210/336 + 196/336 + 192/336) : x = 0 (quy đồng)

7/48 - 193/56 : x  = 0

193/56 : x = 0 + 7/48

193/56 : x = 7/48

              x = 193/56 : 7/48

              x = 1158/49

\(\dfrac{2}{3}\times\left(x+\dfrac{4}{5}\right)=\dfrac{-1}{3}\\ x+\dfrac{4}{5}=\dfrac{-1}{3}:\dfrac{2}{3}\\ x+\dfrac{4}{5}=\dfrac{-1}{3}\times\dfrac{3}{2}\\ x+\dfrac{4}{5}=\dfrac{-1}{2}\\ x=\dfrac{-1}{2}-\dfrac{4}{5}\\ x=\dfrac{-5}{10}-\dfrac{8}{10}\\ x=\dfrac{-13}{10}\)

\(\dfrac{2}{3}.\left(x+\dfrac{4}{5}\right)=-\dfrac{1}{3}\)

    \(\left(x+\dfrac{4}{5}\right)=\left(-\dfrac{1}{3}:\dfrac{2}{3}\right)\)

    \(\left(x+\dfrac{4}{5}\right)=\left(-\dfrac{1}{3}.\dfrac{3}{2}\right)=-\dfrac{1}{2}\)

      \(x\)            \(=\left(-\dfrac{1}{2}\right)-\dfrac{4}{5}\)

      \(x\)            \(=\left(-\dfrac{1}{2}\right)+\left(-\dfrac{4}{5}\right)\)

      \(x\)            \(=-\dfrac{13}{10}\)

\(\frac{3}{2}\)x \(\frac{3}{4}\)x \(\frac{4}{5}\): \(\frac{1}{5}\)= \(\frac{9}{8}\)x \(\frac{4}{5}\): \(\frac{1}{5}\)= \(\frac{9}{10}\): \(\frac{1}{5}\)= \(\frac{9}{10}\)x \(\frac{5}{1}\)= \(\frac{9}{2}\)

bằng 36 phần 8 thì phải

x+\(\frac{1}{3}\)=\(\frac{2}{5}\)- (\(\frac{-1}{3}\))

x + \(\frac{1}{3}\)= \(\frac{2}{5}\)+\(\frac{1}{3}\)

x +1/3 =11/15

x= 11/15 -1/3

x= 2/5

b, 5/7-x=1/4 -(-3/5)

5/7 - x = 1/4 +3/5

5/7 - x =17/20

x = 5/7 -17/ 20

x= -19/140

Bài làm:

Ta có: \(\frac{1}{1.3}+\frac{1}{2.4}+\frac{1}{3.5}+...+\frac{1}{98.100}\)

\(=\left(\frac{1}{1.3}+\frac{1}{3.5}+...+\frac{1}{97.99}\right)+\left(\frac{1}{2.4}+\frac{1}{4.6}+...+\frac{1}{98.100}\right)\)

\(=\frac{1}{2}\left(1-\frac{1}{3}+\frac{1}{3}-\frac{1}{5}+...+\frac{1}{97}-\frac{1}{99}\right)+\frac{1}{2}\left(\frac{1}{2}-\frac{1}{4}+\frac{1}{4}-\frac{1}{6}+...+\frac{1}{98}-\frac{1}{100}\right)\)

\(=\frac{1}{2}\left(1-\frac{1}{99}\right)+\frac{1}{2}\left(\frac{1}{2}-\frac{1}{100}\right)\)

\(=\frac{1}{2}.\frac{98}{99}+\frac{1}{2}.\frac{49}{100}\)

\(=\frac{49}{99}+\frac{49}{200}\)

\(=\frac{14651}{19800}\)