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

=>\(x>19 \)

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