Ta có : 

Đặt \(A=\frac{31}{3}+\frac{31}{15}+...+\frac{31}{143}\)

\(A=\frac{31}{1.3}+\frac{31}{3.5}+...+\frac{31}{11.13}\)

\(\frac{2}{31}A=\frac{2}{1.3}+\frac{2}{3.5}+...+\frac{2}{11.13}\)

\(\frac{2}{31}A=1-\frac{1}{3}+\frac{1}{3}-\frac{1}{5}+...+\frac{1}{11}-\frac{1}{13}\)

\(\frac{2}{31}A=1-\frac{1}{13}\)

\(A=\frac{12}{13}:\frac{2}{31}\)

\(A=\frac{186}{13}\)

\(\Rightarrow x-\frac{186}{13}=\frac{9}{13}\)

\(x=\frac{9}{13}+\frac{186}{13}\)

\(x=\frac{195}{13}=15\)

Ủng hộ mk nha !!! ^_^

\(\text{Đặt }A=\frac{31}{3}+\frac{31}{15}+...+\frac{31}{143}\)

\(\Rightarrow A=\frac{31}{1.3}+\frac{31}{3.5}+...+\frac{31}{11.13}\)

\(\Rightarrow\frac{2}{31}A=\frac{2}{1.3}+\frac{2}{3.5}+...+\frac{2}{11.13}\)

\(\Rightarrow\frac{2}{31}A=1-\frac{1}{3}+\frac{1}{3}-\frac{1}{5}+...+\frac{1}{11}-\frac{1}{13}\)

\(\Rightarrow\frac{2}{31}A=1-\frac{1}{13}\Rightarrow A=\frac{12}{13}:\frac{2}{31}=\frac{186}{13}\)

\(\Rightarrow x-\frac{186}{13}=\frac{9}{13}\)

\(\Rightarrow x=\frac{9}{13}+\frac{186}{13}\)

\(\Rightarrow x=\frac{195}{13}=15\)

b) Thay x=-28

\(\left(-2\right)+17+\left(-28\right)=-13\)

c) Thay x=-4

\(\left(-4\right)+25+\left(-47\right)=-26\)

d) Thay x=8

\(25+8+\left(-13\right)=20\)

\(X-\left(\frac{31}{5}+\frac{31}{15}+\frac{31}{35}+\frac{31}{63}+\frac{31}{99}+\frac{31}{143}\right)=\frac{9}{13}\)

\(X-\left(\frac{31}{5}+\frac{31}{3\cdot5}+\frac{31}{5\cdot7}+\frac{31}{7\cdot9}+\frac{31}{9\cdot11}+\frac{31}{11\cdot13}\right)=\frac{9}{13}\)

\(X-\left[\frac{31}{2}\cdot\left(\frac{1}{3}-\frac{1}{5}+\frac{1}{5}-\frac{1}{7}+\frac{1}{7}-\frac{1}{9}+\frac{1}{9}-\frac{1}{11}+\frac{1}{11}-\frac{1}{13}\right)+\frac{31}{5}\right]=\frac{9}{13}\)

\(X-\left[\frac{31}{2}\cdot\left(\frac{1}{3}-\frac{1}{13}\right)+\frac{31}{5}\right]=\frac{9}{13}\)

\(X-\left[\frac{31}{2}\cdot\frac{10}{39}+\frac{31}{5}\right]=\frac{9}{13}\)

\(X-\frac{1984}{195}=\frac{9}{13}\)

\(\Rightarrow X=\frac{9}{13}+\frac{1984}{195}=\frac{163}{15}\)

163 / 15

mk nghĩ v

bt làm r

\(\frac{31}{3}+\frac{31}{15}+\frac{31}{35}+\frac{31}{63}+\frac{31}{99}+\frac{31}{143}=\frac{31}{1.3}+\frac{31}{3.5}+\frac{31}{5.7}+\frac{31}{7.9}+\frac{31}{9.11}+\frac{31}{11.13}\\ \)

\(=\frac{31}{2}.\left(\frac{2}{1.3}+\frac{2}{3.5}+\frac{2}{5.7}+\frac{2}{7.9}+\frac{2}{9.11}+\frac{2}{11.13}\right)\)

\(=\frac{31}{2}.\left(1-\frac{1}{3}+\frac{1}{3}-\frac{1}{5}+\frac{1}{5}-\frac{1}{7}+\frac{1}{7}-\frac{1}{9}+\frac{1}{9}-\frac{1}{11}+\frac{1}{11}-\frac{1}{13}\right)\)

\(=\frac{31}{2}.\left(1-\frac{1}{13}\right)=\frac{31}{2}.\frac{12}{13}=\frac{31.6}{13}=\frac{186}{13}\)

\(\Rightarrow x-\frac{186}{13}=\frac{9}{13}\Leftrightarrow x=\frac{195}{13}=15\)

\(x-\left(\frac{31}{3}+\frac{31}{15}+\frac{31}{35}+\frac{31}{63}+\frac{31}{99}+\frac{31}{143}\right)=\)\(\frac{9}{13}\)(1)

Đặt \(A=\frac{31}{3}+\frac{31}{15}+\frac{31}{35}+\frac{31}{63}+\frac{31}{99}+\frac{31}{143}\)

\(A=31\left(\frac{1}{3}+\frac{1}{15}+\frac{1}{35}+\frac{1}{63}+\frac{1}{99}+\frac{1}{143}\right)\)

\(\Rightarrow2A=31\left(\frac{2}{3}+\frac{2}{15}+\frac{2}{35}+\frac{2}{63}+\frac{2}{99}+\frac{2}{143}\right)\)

\(2A=31\left(\frac{2}{1.3}+\frac{2}{3.5}+\frac{2}{5.7}+\frac{2}{7.9}+\frac{2}{9.11}+\frac{2}{11.13}\right)\)

\(2A=31\left(2-\frac{1}{3}+\frac{1}{3}-\frac{1}{5}+\frac{1}{5}-\frac{1}{7}+\frac{1}{7}-\frac{1}{9}+\frac{1}{9}-\frac{1}{11}+\frac{1}{11}-\frac{1}{13}\right)\)

\(2A=31\left(2-\frac{1}{13}\right)\)

\(2A=31.\frac{25}{13}\)

\(2A=\frac{775}{13}\)

\(\Rightarrow A=\frac{775}{13}:2\)

\(A=\frac{775}{26}\)

   Thay vào (1) ta có:

 \(x-\frac{775}{26}=\frac{9}{13}\)

\(\Leftrightarrow x=\frac{9}{13}+\frac{775}{26}\)

\(\Leftrightarrow x=\frac{61}{2}\)

ai tích mình ,mình tích lại (ai trên 11 điểm tích mới tích lại)

Giải 
Hiệu số tuổi bố và con không bao giờ thay đổi. 
Hiện nay tuổi con bằng 1/6 tuổi bố. Vậy tuổi bố bằng: 
6/6-1 = 6/5 (hiệu ) 
Sau 4 năm thì tuổi bố bằng: 
4/4-1 = 4/3 ( hiệu ) 
4 năm thì bằng: 
4/3 – 6/5 = 2/15 ( hiệu ) 
Hiệu của tuổi hai bố con là: 
4 : 2/15 = 30 ( tuổi ) 
Tuổi con hiện nay là: 
30 : ( 6 - 1 ) = 6 ( tuổi ) 
Tuổi bố hiện nay là: 
6 x 6 = 36 ( tuổi ) 
Đáp số: 
Con: 6 tuổi 
Bố: 36 tuổi 

\(a,\dfrac{-19}{34}-\left(\dfrac{31}{51}-\dfrac{54}{35}\right)=\dfrac{-19}{34}-\left(-\dfrac{1669}{1785}\right)=\dfrac{79}{210}\\ b,\dfrac{-2}{5}.\dfrac{4}{15}+\left(-0,4\right).\dfrac{26}{15}=-\dfrac{8}{75}-\dfrac{52}{75}=-\dfrac{4}{5}\)

a: x=-11

b: x=4

\(a,\Rightarrow x=-8-3=-11\\ b,\Rightarrow35+x-12=27\\ \Rightarrow x=4\\ c,\Rightarrow2^x=16=2^4\Rightarrow x=4\)

a/ \(3+x=-8\Rightarrow x=-8-3=-11\)

b/ \(\begin{matrix}\left(35+x\right)-12=27\\35+x=27+12\\35+x=39\\x=39-35\\x=4\end{matrix}\)

c/ \(\begin{matrix}2^x+15=31\\2^x=31-15\\2^x=16\\2^x=2^4\\x=4\end{matrix}\)

#)Giải :

\(200-18:\left(372:3x-1\right)-28=166\)

\(\Leftrightarrow200-18:\left(372:3x-1\right)=194\)

\(\Leftrightarrow18:\left(372:3x-1\right)=6\)

\(\Leftrightarrow372:3x-1=3\)

\(\Leftrightarrow3x-1=124\)

\(\Leftrightarrow3x=125\)

\(\Leftrightarrow x=\frac{125}{3}\)

200 - 18 : (372 : 3 . x - 1) - 28 = 166

=> 200 - 18 : (372 : 3.x - 1)     = 166 + 28

=> 200 - 18 : (372 : 3.x) - 1)    = 194

=>          18 : (372 : 3.x - 1)     = 200 - 194

=>          18 : (372 : 3.x - 1)     = 6

=>                  372 : 3.x  - 1      = 18 : 6

=>                  372 : 3.x - 1       = 3

=>                    372 : 3.x          = 3 + 1

=>                    372 : 3.x          = 4

=>                             3.x          = 372 : 4

=>                             3.x          = 93

=>                                x          = 93 : 3

=>                                x          = 31

#)Next:

\(\left(100-99+98-97+...+2-1\right):50+2010-12x=91\)

\(\Leftrightarrow\left[\left(100-99\right)+\left(98-97\right)+...+\left(2-1\right)\right]:50+2010-12x=91\)

\(\Leftrightarrow\left[1+1+...+1\right]:50+2010-12x=91\)

\(\Leftrightarrow50:50+2010-12x=91\)

\(\Leftrightarrow1+2010-12x=91\)

\(\Leftrightarrow2011-12x=91\)

\(\Leftrightarrow12x=1920\)

\(\Leftrightarrow x=160\)