Ta có:

1/30+1/42+1/56+1/72+1/90+1/110+1/132 -x = 2/3

=>1/(5.6) + 1/(6.7) + 1/(7.8) + (1/8.9) + 1/(9.10) + 1/(10.11) + 1/(11.12) - x = 2/3

=>1/5-1/6+1/6-1/7+...+1/11-1/12 - x = 2/3

=>1/5-1/12 - x = 2/3

=>7/60 - x = 2/3

=> x = 7/60 - 2/3

=> x = -11/20

Ta có:

1/30+1/42+1/56+1/72+1/90+1/110+1/132 -x = 2/3

=>1/(5.6) + 1/(6.7) + 1/(7.8) + (1/8.9) + 1/(9.10) + 1/(10.11) + 1/(11.12) - x = 2/3

=>1/5-1/6+1/6-1/7+...+1/11-1/12 - x = 2/3

=>1/5-1/12 - x = 2/3

=>7/60 - x = 2/3

=> x = 7/60 - 2/3

=> x = -11/20

\(\Rightarrow\dfrac{1}{5\times6}+\dfrac{1}{6\times7}+\dfrac{1}{7\times8}+...+\dfrac{1}{11\times12}=\dfrac{21}{x}\\ \Rightarrow\dfrac{1}{5}-\dfrac{1}{6}+\dfrac{1}{6}-\dfrac{1}{7}+...+\dfrac{1}{11}-\dfrac{1}{12}=\dfrac{21}{x}\\ \Rightarrow\dfrac{1}{5}-\dfrac{1}{12}=\dfrac{21}{x}\\ \Rightarrow\dfrac{21}{x}=\dfrac{7}{60}\Rightarrow x=\dfrac{21\cdot60}{7}=180\)

\(\dfrac{1}{30}+\dfrac{1}{42}+\dfrac{1}{56}+\dfrac{1}{72}+\dfrac{1}{90}+\dfrac{1}{110}+\dfrac{1}{132}=\dfrac{21}{x}\)

\(\dfrac{1}{5.6}+\dfrac{1}{6.7}+\dfrac{1}{7.8}+\dfrac{1}{8.9}+\dfrac{1}{9.10}+\dfrac{1}{10.11}+\dfrac{1}{11.12}=\dfrac{21}{x}\)

\(\dfrac{1}{5}-\dfrac{1}{6}+\dfrac{1}{6}-\dfrac{1}{7}+\dfrac{1}{8}-\dfrac{1}{9}+\dfrac{1}{9}-\dfrac{1}{10}+\dfrac{1}{10}-\dfrac{1}{11}+\dfrac{1}{11}-\dfrac{1}{12}=\dfrac{21}{x}\)

\(\dfrac{1}{5}-\dfrac{1}{12}=\dfrac{21}{x}\)

Còn lại bạn tự tính

\(\frac{20}{9}-x=\frac{1}{12}+\frac{1}{20}+.........+\frac{1}{72}\)

\(\Rightarrow\frac{20}{9}-x=\frac{1}{3.4}+\frac{1}{4.5}+.......+\frac{1}{8.9}\)

\(\Rightarrow\frac{20}{9}-x=\frac{1}{3}-\frac{1}{4}+\frac{1}{4}-\frac{1}{5}+........+\frac{1}{8}-\frac{1}{9}\)

\(\Rightarrow\frac{20}{9}-x=\frac{1}{3}-\frac{1}{9}\)

\(\Rightarrow\frac{20}{9}-x=\frac{2}{9}\)

\(\Rightarrow x=\frac{20}{9}-\frac{2}{9}\)

\(\Rightarrow x=\frac{18}{9}=2\)

20/9 -x= 1/3.4 +1/4.5+1/5.6+ 1/6.7+1/7.8+1/8.9
          =1/3+1/4+1/4+1/5+1/5+1/6+1/6+1/7+1/7+1/8+1/8+1/9+1/9
   20/9 -x= 1/3
          x=20/9-1/3
          x= 17/9
Nhớ tích (đúng) cho mình nha

20/9-x=1/12+1/20+1/30+1/42+1/56+1/72 

20/9-x=1/3.4+1/4.5+1/5.6+.............+1/8.9

20/9-x=1/3-1/4+1/4-1/5+................+1/8-1/9

20/9-x=1/3-1/9

20/9-x=2/9

       x=20/9-2/9

       x=18/9

Chọn (k) đúng cho mình nha !!!!!!!!!!!

\(\frac{20}{9}x=\frac{1}{12}+\frac{1}{20}+\frac{1}{30}+\frac{1}{42}+\frac{1}{56}+\frac{1}{72}\)

\(\frac{20}{9}x=\frac{1}{3.4}+\frac{1}{4.5}+\frac{1}{5.6}+\frac{1}{6.7}+\frac{1}{7.8}+\frac{1}{8.9}\)

\(\frac{20}{9}x=\frac{1}{3}-\frac{1}{4}+\frac{1}{4}-\frac{1}{5}+\frac{1}{5}-.....-\frac{1}{9}\)

\(\frac{20}{9}x=\frac{1}{3}-\frac{1}{9}=\frac{2}{9}\)

\(x=\frac{2}{9}:\frac{20}{9}=\frac{1}{10}\)

20/9 . x = 1/12 + 1/20 + 1/30 + 1/42 + 1/56 + 1/72

20/9 . x = 1/3.4 + 1/4.5 + 1/5.6 + 1/6.7 + 1/7.8 + 1/8.9

20/9 . x = 1/3 - 1/4 + 1/4 - 1/5 + ... + 1/8 - 1/9

20/9 . x  = 1/3 - 1/9 = 2/9

x = 2/9 : 20/9 = 2/9 . 9/20 = 1/10

Vậy x = 1/10