a) \(\left|7x-4\right|=-7\)

Mà \(\left|7x-4\right|\ge0\forall x\)

\(\Rightarrow\) phương trình vô nghiệm

b) \(\left|3x-4\right|=\left|7x+5\right|\)

\(\Rightarrow\left[{}\begin{matrix}3x-4=7x+5\\3x-4=-7x-5\end{matrix}\right.\)

\(\Rightarrow\left[{}\begin{matrix}x=-\dfrac{9}{4}\\x=-\dfrac{1}{10}\end{matrix}\right.\)

Vậy ...

tách 

câu a sai đề bn nhé

a: A=2+7x-5=7x-3

b: x<7/3 nên 3x-7<0

=>B=7-3x+2x+3=10-x

\(A=2+7x-5=7x-3\)

\(B=-3x+7+2x+3=-x+10\)

\(A=\dfrac{1}{2}+\dfrac{1}{2^2}+\dfrac{1}{2^3}+...+\dfrac{1}{2^9}\)

\(\Rightarrow2A=1+\dfrac{1}{2}+\dfrac{1}{2^2}+...+\dfrac{1}{2^8}\)

\(\Rightarrow2A-A=1-\dfrac{1}{2^9}\)

\(\Rightarrow A=1-\dfrac{1}{2^9}=\dfrac{511}{512}\)

\(\Rightarrow\left(a+b\right)_{min}=511+512=1023\)

Sửa đề: \(\dfrac{7}{x+5}-\dfrac{x}{5-x}=\dfrac{-x^2}{25-x^2}\)

\(\Leftrightarrow7\left(x-5\right)+x\left(x+5\right)=x^2\)

\(\Leftrightarrow7x-35+5x=0\)

=>12x=35

hay x=35/12

e: 7x<=9x-5

=>7x-9x<=-5

=>-2x<=-5

=>x>=5/2

f: \(\Leftrightarrow7x-5< 8\left(3x-1\right)-4\left(2x+4\right)\)

=>7x-5<24x-8-8x-16

=>7x-5<16x-24

=>-9x<-19

hay x>19/9

e)\(7x≤9x-5 =>7x-9x≤-5 =>-2x≤-5 =>x≥/2\)

=>\(x>19 \)

\(d)5-x-7=x=>2x=-2 =>x=-1\)

49/12 em ạ

49/12 em ạ

e cảm ơn 2 c nhiều ạ 

\(\dfrac{5}{7}=\dfrac{30}{42};\dfrac{6}{7}=\dfrac{36}{42}. Vay5phansocantimla:\dfrac{31}{42};\dfrac{32}{42};\dfrac{33}{42};\dfrac{34}{42};\dfrac{35}{42}\)

Qui đồng 2 phân số lên ta có: 

5/7 = 30/42 và 6/7 = 36/42 

Vậy phân số nằm giữa là: 31/42 ; 32/42; 33/42; 34/42; 35/42

a) 7x - 5 = 3x + 4

   7x - 3x = 4 + 5

    4x = 9

      x = \(\dfrac{9}{4}\) = 2,25

b) |3x| = 2x + 3

     x = \(-\dfrac{3}{5}\) = -0,6

     x = 3

c) \(\dfrac{x+3}{-8}\) \(\ge\) 2

   x + 3 \(\le\) - 16

   x \(\le\) -16 - 3

   x \(\le\) -19