a, Thay x = -2 ; y = 3 ta được 

\(A=\dfrac{4\left(-2\right)-5.3}{8\left(-2\right)-7.3}=\dfrac{-8-15}{-16-21}=\dfrac{23}{37}\)

b, Ta có \(\dfrac{x}{y}=\dfrac{5}{4}\Rightarrow\dfrac{x}{5}=\dfrac{y}{4}=k\Rightarrow x=5k;y=4k\)

Thay vào ta được \(A=\dfrac{4.5k-5.4k}{8.5k-7.4k}=\dfrac{0}{40k-28k}=0\)

Chọn C

BẠN CÓ THỂ GIẢI THÍCH KHÔNG

từ x-y=9=>x=y+9

thay x=y+9 vào B  ta được:

\(\frac{7x-9}{6x+y}=\frac{7\left(y+9\right)-9}{6\left(y+9\right)+y}=\frac{7y+63-9}{6y+54+y}=\frac{7y+54}{\left(6y+y\right)+54}=\frac{7y+54}{7y+54}=1\) (1)

\(\frac{7x+9}{8x-y}=\frac{7\left(y+9\right)+9}{8\left(y+9\right)-y}=\frac{7y+63+9}{8y+72-y}=\frac{7y+72}{\left(8y-y\right)+72}=\frac{7y+72}{7y+72}=1\) (2)

từ (1);(2)

=>B=1-1=0

Vậy B=0

chat vs t

 Ta có x =7  

=>x+1=8

\(\Rightarrow\)\(A=x^{15}-8x^{14}+8x^{13}-8x^{12}+.......8x^2+8x-5\)

\(\Rightarrow x^{15}-\left(x+1\right)x^{14}+\left(x+1\right)x^{13}-\left(x+1\right)x^{12}+...\left(x+1\right)x^2\)

\(+\left(x+1\right)x^5\)

\(\Rightarrow x^{15}-x^{15}-x^{14}+x^{14}+x^{13}-x^{13}-x^{12}+...-x^3-x^2+x-5\)

\(\Rightarrow x-5\Leftrightarrow A=7-5=2\Rightarrow A=2\)

Vậy A=2 khi x=7

\(A=\dfrac{2x^2\left(3x-4y+2\right)}{x\left(3x+y\right)\left(3x-y\right)}=\dfrac{2x\left(3x-4y+2\right)}{\left(3x+y\right)\left(3x-y\right)}\\ A=\dfrac{2\left(3-8+2\right)}{\left(3+2\right)\left(3-2\right)}=\dfrac{2\left(-3\right)}{5}=\dfrac{-6}{5}\)

a, ĐKXĐ: x≠±3

A=\(\left(\dfrac{3-x}{x+3}.\dfrac{x^2+6x+9}{x^2-9}+\dfrac{x}{x+3}\right):\dfrac{3x^2}{x+3}\)

A=\(\left(\dfrac{3-x}{x+3}.\dfrac{\left(x+3\right)^2}{\left(x+3\right)\left(x-3\right)}+\dfrac{x}{x+3}\right):\dfrac{3x^2}{x+3}\)

A=\(\left(\dfrac{3-x}{x-3}+\dfrac{x}{x+3}\right):\dfrac{3x^2}{x+3}\)

A=\(\left(\dfrac{9-x^2}{x^2-9}+\dfrac{x^2-3x}{x^2-9}\right):\dfrac{3x^2}{x+3}\)

A=\(\left(\dfrac{-3}{x+3}\right):\dfrac{3x^2}{x+3}\)

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

b, Thay x=\(-\dfrac{1}{2}\) (TMĐKXĐ) vào A ta có:

\(\dfrac{-1}{\left(-\dfrac{1}{2}\right)^2}\)=-4

c, A<0 ⇔ \(\dfrac{-1}{x^2}< 0\) ⇔ x²>0 (Đúng với mọi x)

Vậy để A<0 thì x đúng với mọi giá trị (trừ ±3)

Đáp án B.