\(\frac{y+z}{x}=\frac{x+z}{y}=\frac{x+y}{z}\Rightarrow k=2\Rightarrow x=y=z=1\)

A=6

\(\frac{x-y-z}{x}=1-\frac{y+z}{x}\) tương tự con khác

=> x=y=z

=> A=6

sorry mấy bạn =x+y+z chứ ko phải =x+y=z :P 

Áp dụng dãy tỉ số bằng nhau ta có:

\(\frac{x}{z+y+1}=\frac{y}{z+x+1}=\frac{z}{x+y+1}=x+y+z=\frac{x+y+z}{2\left(x+y+z\right)+3}\)

\(\Rightarrow\left(x+y+z\right)\left(1-\frac{1}{2\left(x+y+z\right)+3}\right)=0\)

\(\Rightarrow\orbr{\begin{cases}x+y+z=0\\2\left(x+y+z\right)+3=1\end{cases}\Rightarrow\orbr{\begin{cases}x+y+z=0\\x+y+z=-1\end{cases}}}\)

Vậy mọi số x,y,z thỏa mãn \(\orbr{\begin{cases}x+y+z=0\\x+y+z=-1\end{cases}}\) đều thỏa mãn bài toán

cứ sai sai kiểu gì đây

sao ( x+y+x)(1-1/2(x+y+z)+3)= 0 ha ban.. mk thay cu sai sai... o cho 1-1/2(x+y+z)+3

\(\dfrac{y+z-x}{x}=\dfrac{z+x-y}{y}=\dfrac{x+y-z}{z}\\ \Rightarrow\dfrac{y+z-x}{x}+2=\dfrac{z+x-y}{y}+2=\dfrac{x+y-z}{z}+2\\ \Rightarrow\dfrac{x+y+z}{x}=\dfrac{x+y+z}{y}=\dfrac{x+y+z}{z}\\ \Rightarrow x=y=z\\ \Rightarrow A=\left(1+1\right).\left(1+1\right).\left(1+1\right)=8\)

y+z+1+x+z+2+x+y-3/x+y+z=2(x+y+z)/x+y+z=2

nên x+y+z=1:2=0,5 suy ra x+y+z/2=0,5:2=1/4

Đặt: \(\frac{x-y}{z}+\frac{y-z}{x}+\frac{z-x}{y}=M\)

Ta có: 

\(M\cdot\frac{z}{x-y}=1+\frac{z}{x-y}\cdot\left(\frac{y-z}{x}+\frac{z-x}{y}\right)=1+\frac{z}{x-y}\cdot\frac{y^2-yz+xz-x^2}{xy}\)

\(=1+\frac{z}{x-y}\cdot\frac{\left(x-y\right)\left(z-x-y\right)}{xy}=1+\frac{2z^2}{xyz}=1+\frac{2z^3}{xyz}\)            (1)

Tương tự ta cũng có:

\(M\cdot\frac{x}{y-z}=1+\frac{2x^3}{xyz}\)              (2)

\(M\cdot\frac{y}{z-x}=1+\frac{2y^3}{xyz}\)            (3)

Từ (1);(2);(3) suy ra

\(M\cdot\left(\frac{z}{x-y}+\frac{x}{y-z}+\frac{y}{z-x}\right)=3+\frac{2\left(x^3+y^3+z^3\right)}{xyz}\)

Mà \(x+y+z=0\Rightarrow x^3+y^3+z^3=3xyz\)

Nên:

\(M\cdot\left(\frac{z}{x-y}+\frac{x}{y-z}+\frac{y}{z-x}\right)=3+\frac{2\cdot3xyz}{xyz}=9\)

=>đpcm