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18 tháng 7 2016

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

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

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

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

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

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

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

18 tháng 7 2016

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

5 tháng 5 2016

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

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

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

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

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

5 tháng 5 2016

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

23 tháng 6 2018

1/12+1/6+1/2=(1+2+6)/12=9/12=3/4

1/30+1/20=(3+2)/60=5/6=1/12

1/56+1/42=1/7(1/8+1/6)=1/7(3+4)/24=1/24

8/9-1/72=(8.8-1)/72=63/72=7/8

1/12+1/24=(2+1)/24=3/4

3/4-3/4=0

k cho mik nha!Chúc bn học tốt

6 tháng 11 2021

Ta có 8/9 - 1/72 - 1/56 - 1/42 - 1/30 - 1/20 - 1/12 - 1/6 - 1/2


= 8/9 -(1/72 +1/56 +1/42 + 1/30 + 1/20 + 1/12 + 1/6 + 1/2)


= 8/9 - (1/9.8 + 1/8.7 + ...+ 1/3.2 + 1/2.1)


= 8/9 - (1 - 1/2 + 1/2 - 1/3 +....+ 1/8 - 1/9)


= 8/9 - (1 - 1/9)


= 8/9 - 8/9


= 0

21 tháng 7 2015

9/10-1/90-1/72-1/56-1/42-1/30-1/20-1/12-1/6-1/2

=9/10-(1/9*10+1/8*9+...+1/1*2)

=9/10-(1/9-1/10+...+1-1/2)

=9/10-(-1/10+1)=9/10-9/10=0

= 9/1.10 + 1/9.10 + 1/8.9 + 1/7.8 + 1/6.7 +1/5.6 + 1/4.5 +1/3.4 +1/2.3 + 1/1.2

= 1/1.2 + 1/2.3 + 1/3.4 + ... + 9/1.10 ( viết ngược lại)

= 1-1/2 + 1/2  -1/3 + 1/3 +....-1/10

= 1 - 1/10

= 9/10

26 tháng 4 2018

\(\frac{20}{9}-x=\frac{1}{3.4}+\frac{1}{4.5}+...+\frac{1}{8.9}=\frac{1}{3}-\frac{1}{4}+...+\frac{1}{8}-\frac{1}{9}=\frac{1}{3}-\frac{1}{9}=\frac{2}{9}\)

\(\Rightarrow\frac{20}{9}-x=\frac{2}{9}\Rightarrow x=\frac{20}{9}-\frac{2}{9}=\frac{18}{9}=2\)

Vậy x = 2.

k cho mk nha

\(2\frac{2}{9}-x=\frac{1}{12}+\frac{1}{20}+\frac{1}{30}+\frac{1}{42}+\frac{1}{56}+\frac{1}{72}\)

\(2\frac{2}{9}-x=\frac{1}{3\cdot4}+\frac{1}{4\cdot5}+\frac{1}{5\cdot6}+\frac{1}{6\cdot7}+\frac{1}{7\cdot8}+\frac{1}{8\cdot9}\)

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

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

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

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

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

\(x=2\)

Vậy x = 2

26 tháng 4 2018

\(2\dfrac{2}{9}-x=\dfrac{1}{12}+\dfrac{1}{20}+\dfrac{1}{30}+\dfrac{1}{42}+\dfrac{1}{56}+\dfrac{1}{72}\)

\(\Rightarrow2\dfrac{2}{9}-x=\dfrac{1}{3.4}+\dfrac{1}{4.5}+...+\dfrac{1}{7.8}+\dfrac{1}{8.9}\)

\(\Rightarrow2\dfrac{2}{9}-x=\dfrac{1}{3}-\dfrac{1}{4}+\dfrac{1}{4}-\dfrac{1}{5}+...+\dfrac{1}{7}-\dfrac{1}{8}+\dfrac{1}{8}-\dfrac{1}{9}\)

\(\Rightarrow2\dfrac{2}{9}-x=\dfrac{1}{3}-\dfrac{1}{9}\)

\(\Rightarrow2\dfrac{2}{9}-x=\dfrac{2}{9}\)

\(\Rightarrow x=2\dfrac{2}{9}-\dfrac{2}{9}\)

\(\Rightarrow x=2\)

Vậy x=2

26 tháng 4 2018

2\(\dfrac{2}{9}\) - x = \(\dfrac{1}{3\cdot4}\)+\(\dfrac{1}{4\cdot5}\)+\(\dfrac{1}{5\cdot6}\)+\(\dfrac{1}{6\cdot7}\)+\(\dfrac{1}{7\cdot8}\)+\(\dfrac{1}{8\cdot9}\)

2\(\dfrac{2}{9}\)-x = \(\dfrac{1}{3}\)-\(\dfrac{1}{4}\)+\(\dfrac{1}{4}\)+...+\(\dfrac{1}{8}\)-\(\dfrac{1}{9}\)

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

2\(\dfrac{2}{9}\)-x = \(\dfrac{9}{27}\)- \(\dfrac{3}{27}\)

2\(\dfrac{2}{9}\)-x = \(\dfrac{2}{9}\)

\(\dfrac{20}{9}\) -x = \(\dfrac{2}{9}\)

x = \(\dfrac{20}{9}-\dfrac{2}{9}\)

x = 2

Vậy x = 2

6 tháng 4 2017

Link này bạn:Câu hỏi của Hoàng Hà Nhi - Toán lớp 6 | Học trực tuyến

6 tháng 4 2017

\(\dfrac{x-1}{12}+\dfrac{x-1}{20}+\dfrac{x-1}{30}+...+\dfrac{x-1}{72}=\dfrac{16}{9}\\ \left(x-1\right)\left(\dfrac{1}{12}+\dfrac{1}{20}+\dfrac{1}{30}+...+\dfrac{1}{72}\right)=\dfrac{16}{9}\\ \left(x-1\right)\left(\dfrac{1}{3.4}+\dfrac{1}{4.5}+\dfrac{1}{5.6}+...+\dfrac{1}{8.9}\right)=\dfrac{16}{9}\\ \left(x-1\right)\left(\dfrac{1}{3}-\dfrac{1}{4}+\dfrac{1}{4}-\dfrac{1}{5}+...+\dfrac{1}{8}-\dfrac{1}{9}\right)=\dfrac{16}{9}\\ \left(x-1\right)\left(\dfrac{1}{3}-\dfrac{1}{9}\right)=\dfrac{16}{9}\\ \left(x-1\right)\dfrac{2}{9}=\dfrac{16}{9}\\ x-1=8\\ x=8+1\\ x=9\)

24 tháng 2 2017

8/9 - 1/72 - 1/56 - 1/42 - 1/30 - 1/20 - 1/12 - 1/6 - 1/2 = 0

24 tháng 2 2017

= 0 nha bạn. chọn mình nha