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

=5/36

đúng thì k mình nhé 

làm ơn giúp mình mình đang bị âm

A = 1/20 +   1/30 + 1/42   +   1/56  + 1/72

A= 1/4x5 + 1/5x6 + 1/6x7 + 1/7x8 + 1/8x9

A= 1/4 - 1/5 + ......................+ 1/8 - 1/9

A= 1/4 - 1/9

A=     5/36

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

\(A=\frac{1}{4.5}+\frac{1}{5.6}+...+\frac{1}{7.8}+\frac{1}{8.9}\)

\(A=\frac{1}{4}-\frac{1}{5}+\frac{1}{5}-\frac{1}{6}+...+\frac{1}{8}-\frac{1}{9}\)

\(A=\frac{1}{4}-\frac{1}{9}\)

\(\Rightarrow A=\frac{5}{36}\)

(-1/20)+(-1/30)+(-1/42)+(-1/56)+(-1/72)+(-1/90)= - 3/20

\(\frac{-1}{20}+\frac{-1}{30}+\frac{-1}{42}+\frac{-1}{56}+\frac{-1}{72}+\frac{-1}{90}\)

= \(\frac{-1}{4.5}+\frac{-1}{5.6}+\frac{-1}{6.7}+\frac{-1}{7.8}+\frac{-1}{8.9}+\frac{-1}{9.10}\)

= \(\left(-1\right).\left(\frac{1}{4.5}+\frac{1}{5.6}+\frac{1}{6.7}+\frac{1}{7.8}+\frac{1}{8.9}+\frac{1}{9.10}\right)\)

= \(\left(-1\right).\left(\frac{1}{4}-\frac{1}{5}+\frac{1}{5}+\frac{1}{6}+\frac{1}{6}-\frac{1}{7}+\frac{1}{7}-\frac{1}{8}+\frac{1}{8}-\frac{1}{9}+\frac{1}{9}-\frac{1}{10}\right)\)

= \(\left(-1\right).\left(\frac{1}{4}-\frac{1}{10}\right)\)

= \(\left(-1\right).\frac{3}{20}\)

= \(\frac{-3}{20}\)

đặt dấu âm ra ngoài

=-(1/20+1/30+1/42+..........1/90)

=-(1/4.5+1/5.6+1/6.7+......1/9.10)

=-(1/4-1/5+1/5-1/6+.....+1/9-1/10)

=-(1/4-1/10)

=-0,15 

1/20+1/30+1/42+1/56+1/72+1/90 = 3/20

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

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

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

= 1/4 - 1/10

= 3/20

6323121356214488888888888888888888888888888888888888888888888888888888888888888888

\(\frac{1}{20}+\frac{1}{30}+\frac{1}{42}+\frac{1}{56}+\frac{1}{72}+\frac{1}{90}\)

\(=\frac{1}{4\cdot5}+\frac{1}{5\cdot6}+\frac{1}{6\cdot7}+\frac{1}{7\cdot8}+\frac{1}{8\cdot9}+\frac{1}{9\cdot10}\)

\(=\frac{1}{4}-\frac{1}{5}+\frac{1}{5}-\frac{1}{6}+...+\frac{1}{9}-\frac{1}{10}\)

\(=\frac{1}{4}-\frac{1}{10}=\frac{5}{20}-\frac{2}{20}=\frac{3}{20}\)

\(\frac{1}{20}\)+ \(\frac{1}{30}\)+ \(\frac{1}{42}\)+ \(\frac{1}{56}\)+ \(\frac{1}{72}\)+ \(\frac{1}{90}\)

= \(\frac{1}{4.5}\)+ \(\frac{1}{5.6}\)+ \(\frac{1}{6.7}\)+ \(\frac{1}{7.8}\)+ \(\frac{1}{8.9}\)+ \(\frac{1}{9.10}\)

= \(\frac{1}{4}\)- \(\frac{1}{5}\)+ \(\frac{1}{5}\)- \(\frac{1}{6}\)+ .... + \(\frac{1}{9}\)- \(\frac{1}{10}\)

= \(\frac{1}{4}\)- \(\frac{1}{10}\)

= \(\frac{5}{20}\)- \(\frac{2}{20}\)

=   \(\frac{3}{20}\)