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+ Nếu x + y + z = 0 => x + y = -z; y + z = -x; x + z = -y
A = (1 + y/x)(1 + z/y)(1 + x/z)
A = (x+y)/x . (y+z)/y . (x+z)/z
A = -z/x . (-x)/y . (-y)/z = -1
+ Nếu x + y + z khác 0
x-y-z/x = -x+y-z/y = -x-y+z/z
<=> 1 - (y+z)/x = 1 - (x+z)/y = 1 - (x+y)/z
<=> y+z/x = x+z/y = x+y/z
Áp dụng t/c của dãy tỉ số = nhau ta có:
y+z/x = x+z/y = x+y/z = 2(x+y+z)/x+y+z = 2
A = (x+y)/x . (y+z)/y . (x+z)/z = 8
\(\frac{x-y-z}{x}=\frac{y-x-z}{y}=\frac{z-x-y}{z}=\frac{x-y-z+y-x-z+z-x-y}{x+y+z}=\frac{-x-y-z}{x+y+z}=-1\)
\(\rightarrow\begin{cases}x-y-z=-x\\y-x-z=-y\\z-x-y=-z\end{cases}\)
\(\leftrightarrow\begin{cases}y+z=2x\\z+x=2y\\x+y=2z\end{cases}\)
\(A=\frac{x+y}{z}.\frac{y+z}{x}.\frac{z+x}{y}=8\)
đặt \(\frac{x-y}{z}=a;\frac{y-z}{x}=b;\frac{z-x}{y}=c\)
\(\Rightarrow\)\(\frac{z}{x-y}=\frac{1}{a};\frac{x}{y-z}=\frac{1}{b};\frac{y}{z-x}=\frac{1}{c}\)
Ta có : \(A=\left(a+b+c\right)\left(\frac{1}{a}+\frac{1}{b}+\frac{1}{c}\right)\)
\(A=1+\frac{b}{a}+\frac{c}{a}+\frac{a}{b}+1+\frac{c}{b}+\frac{a}{c}+\frac{b}{c}+1=3+\frac{b+c}{a}+\frac{a+c}{b}+\frac{a+b}{c}\)
Ta có : \(\frac{b+c}{a}=\left(b+c\right)\frac{1}{a}=\left(\frac{y-z}{x}+\frac{z-x}{y}\right)\frac{z}{x-y}=\frac{y^2-yz+xz-x^2}{xy}.\frac{z}{x-y}=\frac{\left(y-x\right)\left(x+y-z\right)}{xy}.\frac{z}{x-y}=\frac{\left(z-x-y\right)z}{xy}=\frac{2z^2}{xy}\)vì x + y + z = 0 \(\Rightarrow\)z = -x - y
Tương tự : \(\frac{a+c}{b}=\frac{2x^2}{yz}\); \(\frac{a+b}{c}=\frac{2y^2}{xz}\)
\(\frac{b+c}{a}+\frac{a+c}{b}+\frac{a+b}{c}=\frac{2z^2}{xy}+\frac{2x^2}{yz}+\frac{2y^2}{xz}=\frac{2\left(x^3+y^3+z^3\right)}{xyz}=\frac{2.3xyz}{xyz}=6\)( vì x + y + z = 0 \(\Rightarrow\)x3 + y3 + z3 = 3xyz )
Vậy A = 3 + 6 = 9
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
\(\frac{y+z}{x}=\frac{x+z}{y}=\frac{x+y}{z}\Rightarrow k=2\Rightarrow x=y=z=1\)
A=6
\(\frac{x-y-z}{x}=1-\frac{y+z}{x}\) tương tự con khác
=> x=y=z
=> A=6