Bài 1: 

\(\Leftrightarrow\left(\dfrac{1}{11}-\dfrac{1}{21}\right)\cdot462-\left[2.04:\left(x+1.05\right)\right]:0.12=19\)

\(\Leftrightarrow\left[2.04:\left(x+1.05\right)\right]:0.12=1\)

\(\Leftrightarrow2.04:\left(x+1.05\right)=0.12\)

\(\Leftrightarrow x+1.05=17\)

hay x=15,85

=> 1/11 - 1/13 + 1/13 - 1/15 + ..... + 1/19 - 1/21 - x + 4 + 221/231 = 7/3

=> 1/11 - 1/21 - x + 4 + 221/231 = 7/3

=> 2099/420 - x = 7/3

=> x = 2099/420 - 7/3 = 373/140

Tk mk nha

Bài làm

\(\frac{2}{11.13}+\frac{2}{13.15}+...+\frac{2}{19.21}-x+4+\frac{221}{231}=\frac{7}{3}\)

\(\Leftrightarrow2\left(\frac{1}{11.13}+\frac{1}{13.15}+...+\frac{1}{19.21}\right)-x+4+\frac{221}{231}=\frac{7}{3}\)

\(\Leftrightarrow2\left(\frac{1}{11}-\frac{1}{13}+\frac{1}{13}-\frac{1}{15}+...+\frac{1}{19}-\frac{1}{21}\right)-x+4+\frac{221}{231}=\frac{7}{3}\)

\(\Leftrightarrow2\left(\frac{1}{11}-\frac{1}{21}\right)-x+4+\frac{221}{231}=\frac{7}{3}\)

\(\Leftrightarrow2.\frac{10}{231}-x+4+\frac{221}{231}=\frac{7}{3}\)

\(\Leftrightarrow\frac{20}{231}-x+4+\frac{221}{231}=\frac{7}{3}\)

\(\Leftrightarrow\frac{20}{231}-x+\frac{924}{231}+\frac{221}{231}=\frac{539}{231}\)

\(\Leftrightarrow\frac{20}{231}-x+\frac{924}{231}=\frac{539}{231}-\frac{221}{231}\)

\(\Leftrightarrow\frac{20}{231}-x+\frac{924}{231}=\frac{318}{231}\)

\(\Leftrightarrow\frac{20}{231}-x=\frac{318}{231}-\frac{924}{231}\)

\(\Leftrightarrow\frac{20}{231}-x=-\frac{606}{231}\)

\(\Leftrightarrow x=\frac{20}{231}-\frac{606}{231}\)

\(\Leftrightarrow x=-\frac{586}{231}\)

Vậy \(\Leftrightarrow=-\frac{586}{231}\)

ta có : 2/11*15 +2/13*15+...+2/19*21 - x +4+221/231=7/3

=>(1/11 -1/13 + 1/13-1/15+...+1/19 - 1/21)-x+4 +221/231=7/3

<=>(1/11 - 1/21)-x+4 + 221/231=7/3

<=>10/231 -  x+4 +221/231 =7/3

<=>1-x +4=7/3

<=>x+4=1-7/3

<=>x+4=-4/3

<=>x=-4/3-4

<=>x=-16/3

vậy x=-16/3

\(\left(\frac{2}{11.13}+\frac{2}{13.15}+.........+\frac{2}{19.21}\right)x623-24,808:\left(y+0,5\right)+12,04=30\)

Ta tính bên trong ngoặc:

\(\frac{2}{11.13}+\frac{2}{13.15}+..........+\frac{2}{19.21}\)

= \(\frac{2}{11}-\frac{2}{13}+\frac{2}{13}-\frac{2}{15}+...........+\frac{2}{19}-\frac{2}{21}\)

= \(\frac{1}{11}-\frac{1}{13}+\frac{1}{13}-\frac{1}{15}+.........+\frac{1}{19}-\frac{1}{21}\)

=> \(\frac{1}{11}-\frac{1}{19}\)= \(\frac{8}{209}\)

=> \(\frac{8}{209}x623-24,808:\left(y+0,5\right)+12,04=30\)

      Tiếp tục làm tiếp nhé

Nhầm mk làm lại:

\(\frac{1}{11}-\frac{1}{21}=\frac{10}{231}\)

=> \(\frac{10}{231}x623-24,808:\left(y+05\right)+12,04=30\)

      \(\frac{890}{33}\)- 24,808 : .......

Làm tiếp đi nhé

Bài 2: ta có tích riêng thứ nhất là .....5, thứ hai cũng là ....5 -> chữ số tận cùng là: ....5 - ....5 = ...0

Bài 3: Gọi số có hai chữ số đó là ab (a,b =<9) 
...........................__..... _ 
Theo đề bài ta có: ab = 9b 
=> b = (2; 3; 4; 5; 6; 7; 8; 9) 
.......................................... 
=> Tương ứng với b ta có ab = (18; 27; 36; 45; 54; 63; 72; 81) 
Nhận xét: Chỉ có 45 = 9.5 
Vậy số đó là 45

Bạn nào giúp mik bài 1 với bài 4 đi

Sửa đề : \(\frac{1}{21}+\frac{1}{28}+\frac{1}{36}+...+\frac{2}{x\left(x+1\right)}=\frac{2}{9}\)

\(\Leftrightarrow\)\(\frac{2}{42}+\frac{2}{56}+\frac{2}{72}+...+\frac{2}{x\left(x+1\right)}=\frac{2}{9}\)

\(\Leftrightarrow\)\(2\left(\frac{1}{42}+\frac{1}{56}+\frac{1}{72}+...+\frac{1}{x\left(x+1\right)}\right)=\frac{2}{9}\)

\(\Leftrightarrow\)\(2\left(\frac{1}{6.7}+\frac{1}{7.8}+\frac{1}{8.9}+...+\frac{1}{x\left(x+1\right)}\right)=\frac{2}{9}\)

\(\Leftrightarrow\)\(2\left(\frac{1}{6}-\frac{1}{7}+\frac{1}{7}-\frac{1}{8}+\frac{1}{8}-\frac{1}{9}+...+\frac{1}{x}-\frac{1}{x+1}\right)=\frac{2}{9}\)

\(\Leftrightarrow\)\(2\left(\frac{1}{6}-\frac{1}{x+1}\right)=\frac{2}{9}\)

\(\Leftrightarrow\)\(\frac{1}{6}-\frac{1}{x+1}=\frac{2}{9}.\frac{1}{2}\)

\(\Leftrightarrow\)\(\frac{1}{6}-\frac{1}{x+1}=\frac{1}{9}\)

\(\Leftrightarrow\)\(\frac{1}{x+1}=\frac{1}{6}-\frac{1}{9}\)

\(\Leftrightarrow\)\(\frac{1}{x+1}=\frac{1}{18}\)

\(\Leftrightarrow\)\(x+1=18\)

\(\Leftrightarrow\)\(x=18-1\)

\(\Leftrightarrow\)\(x=17\)

Vậy \(x=17\)

Chúc bạn học tốt ~ 

\(x-\frac{20}{11.13}-\frac{20}{13.15}-\frac{20}{15.17}-...-\frac{20}{53.55}=\frac{3}{11}\)

\(\Leftrightarrow\)\(x+10\left(\frac{2}{11.13}+\frac{2}{13.15}+\frac{2}{15.17}+...+\frac{2}{53.55}\right)=\frac{3}{11}\)

\(\Leftrightarrow\)\(x+10\left(\frac{1}{11}-\frac{1}{13}+\frac{1}{13}-\frac{1}{15}+\frac{1}{15}-\frac{1}{17}+...+\frac{1}{53}-\frac{1}{55}\right)=\frac{3}{11}\)

\(\Leftrightarrow\)\(x+10\left(\frac{1}{11}-\frac{1}{55}\right)=\frac{3}{11}\)

\(\Leftrightarrow\)\(x+10.\frac{4}{55}=\frac{3}{11}\)

\(\Leftrightarrow\)\(x+\frac{40}{55}=\frac{3}{11}\)

\(\Leftrightarrow\)\(x=\frac{3}{11}-\frac{40}{55}\)

\(\Leftrightarrow\)\(x=\frac{-5}{11}\)

Vậy \(x=\frac{-5}{11}\)

Chúc bạn học tốt ~ 

\(\frac{1}{21}+\frac{1}{28}+\frac{1}{36}+...+\frac{1}{x\left(x+1\right)}=\frac{2}{9}\)

\(\Rightarrow\frac{2}{42}+\frac{2}{56}+\frac{2}{72}+...+\frac{1}{x\left(x+1\right)}=\frac{2}{9}\)

\(\Rightarrow\frac{2}{6\cdot7}+\frac{2}{7\cdot8}+\frac{2}{8\cdot9}+...+\frac{1}{x\left(x+1\right)}=\frac{2}{9}\)

\(\Rightarrow2\left(\frac{1}{6\cdot7}+\frac{1}{7\cdot8}+\frac{1}{8\cdot9}+...+\frac{1}{x\left(x+1\right)}\right)=\frac{2}{9}\)

\(\Rightarrow2\left(\frac{1}{6}-\frac{1}{7}+\frac{1}{7}-\frac{1}{8}+\frac{1}{8}-\frac{1}{9}+...+\frac{1}{x}-\frac{1}{x+1}\right)=\frac{2}{9}\)

\(\Rightarrow2\left(\frac{1}{6}-\frac{1}{x+1}\right)=\frac{2}{9}\)

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

\(\Rightarrow\frac{2}{x+1}=\frac{1}{9}\)

=> 2.9 = x + 1

=> x + 1 = 18

=> x = 17