em lớp 6  nha

B= 1/2 + 1/6 + 1/12 +1/20 + 1/30 + 1/42 + 1/56 + 1/72

B= 1/1*2 + 1/2*3 + 1/3*4 + 1/4*5 + 1/5*6 + 1/6*7 + 1/7*8 + 1/8*9

B=1-1/2+1/2-1/3+1/3-1/4+1/4-1/5+1/5-1/6+1/6-1/7+1/7-1/8+1/8-1/9

B=1+0-0-0-0-0-0-0-1/9

B=1-1/9

B=8/9

k em nha

phần A em chịu

còn câu dưới đó e

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

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

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

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

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

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

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

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

\(=\frac{1}{3.4}+\frac{1}{4.5}+\frac{1}{\cdot5.6}+\frac{1}{6.7}+\frac{1}{7.8}+\frac{1}{8.9}\)
=\(\left(\frac{1}{3}-\frac{1}{4}\right)+\left(\frac{1}{4}-\frac{1}{5}\right)+\left(\frac{1}{5}-\frac{1}{6}\right)+\left(\frac{1}{6}-\frac{1}{7}\right)+\left(\frac{1}{7}-\frac{1}{8}\right)+\left(\frac{1}{8}-\frac{1}{9}\right)\)
=1/3-1/9=2/9
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li-ke cho mk nha bn Vũ lệ Quyên

\(\frac{1}{12}+\frac{1}{20}+\frac{1}{30}+\frac{1}{42}+\frac{1}{56}+\frac{1}{72}=\frac{1}{3\times4}+\frac{1}{4\times5}+\frac{1}{5\times6}+\frac{1}{6\times7}+\frac{1}{7\times8}+\frac{1}{8\times9}\)

\(=\frac{1}{3}-\frac{1}{4}+\frac{1}{4}-\frac{1}{5}+\frac{1}{5}-\frac{1}{6}+\frac{1}{6}-\frac{1}{7}+\frac{1}{7}-\frac{1}{8}+\frac{1}{8}-\frac{1}{9}\)

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

\(\frac{1}{6}+\frac{1}{12}+\frac{1}{20}+\frac{1}{30}+\frac{1}{42}+\frac{1}{56}+\frac{1}{72}\)

\(=\frac{1}{2\cdot3}+\frac{1}{3\cdot4}+\frac{1}{4\cdot5}+\frac{1}{5\cdot6}+\frac{1}{6\cdot7}+\frac{1}{7\cdot8}+\frac{1}{8\cdot9}\)

\(=\frac{1}{2}-\frac{1}{3}+\frac{1}{3}-\frac{1}{4}+\frac{1}{4}-\frac{1}{5}+\frac{1}{5}-\frac{1}{6}+\frac{1}{6}-\frac{1}{7}+\frac{1}{7}-\frac{1}{8}+\frac{1}{8}-\frac{1}{9}\)

\(=\frac{1}{2}-\frac{1}{9}\)

\(=\frac{7}{18}\)

1/12 + 1/20 1/30  + 1/42 + 1/56 + 1/72 + 1/90

= 1/3.4 + 1/4.5 + 1/5.6 + 1/6.7 + 1/7.8 + 1/8.9 + 1/9.10

= 1/3 - 1/4 + 1/4 - 1/5 + 1/5 - 1/6 + 1/6 - 1/7 + 1/7 - 1/7 + 1/8 - 1/9 + 1/9 - 1/10

= 1/3 - 1/10

= 7/30

1/12+1/20+1/30+1/42+1/56+1/72+1/90=7/30

\(=\frac{7}{30}\)nhé bạn

1/2+1/6+...+1/72=1/1x2+1/2x3+...+1/8x9=1/1-1/2+1/2-1/3+...+1/8-1/9=1-1/9=8/9

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

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

\(=\frac{1}{3}-\frac{1}{4}+\frac{1}{4}-\frac{1}{5}+\frac{1}{5}-\frac{1}{6}+\frac{1}{6}-\frac{1}{7}+\frac{1}{7}-\frac{1}{8}+\frac{1}{8}-\frac{1}{9}\)

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

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

\(=\frac{2}{9}\)

= 1/3 x 4   +   1 / 4 x 5   +    1 / 5x6   +   1 / 6 x 7   +    1 / 7 x 8    +   1 / 8 x 9 

= 1/3 -   1 / 4  + 1 /4 - 1/5  + 1/5 - 1/6 + ................... + 1/8 - 1/9

= 1/3 - 1/9 

= 2/9

\(=\frac{1}{3\times4}+\frac{1}{4\times5}+...+\frac{1}{8\times9}\)

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

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

\(=\frac{2}{9}\)

A=1/2.3+1/3.4+1/4.5+1/5.6+1/6.7+1/7.8+1/8.9+1/9.10

A=1/2-1/3+1/3-1/4+1/4-1/5+...+1/9-1/10

A=1/2-1/10

A=2/5