cho x+y+z=0
Tính GTBT: xy/(x^2+y^2-z^2)+yz/(y^2+z^2-x^2)+zx/(z^2+x^2-y^2)
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29 tháng 9 2023
`@ x+y+z=1`.
`<=>` \(\left\{{}\begin{matrix}x=1-y-z\\y=1-z-x\\z=1-x-y\end{matrix}\right.\)
`P=(x+y)^2/(xy+1-x-y).(y+z)^2/(yz-y-z+1).(x+z)^2/(xy-x-y+1)`.
`<=> ((1-z)^2(1-y)^2(1-x)^2)/((1-x)(1-y)(1-y)(1-z)(1-z)(1-x).`
`=1.`
Vậy `P` không phụ thuộc vào giá trị của biến.
29 tháng 9 2023
`@ x+y+z=1`.
`<=>` \(\left\{{}\begin{matrix}x=1-y-z\\y=1-z-x\\z=1-x-y\end{matrix}\right.\)
`P=(x+y)^2/(xy+1-x-y).(y+z)^2/(yz-y-z+1).(x+z)^2/(xy-x-y+1)`.
`<=> ((1-z)^2(1-y)^2(1-x)^2)/((1-x)(1-y)(1-y)(1-z)(1-z)(1-x).`
`=1.`
Vậy `P` không phụ thuộc vào giá trị của biến.
20 tháng 11 2021
\(\dfrac{xy}{x+y}=\dfrac{yz}{y+z}=\dfrac{zx}{z+x}\\ \Rightarrow\dfrac{x+y}{xy}=\dfrac{y+z}{yz}=\dfrac{z+x}{zx}\\ \Rightarrow\dfrac{1}{y}+\dfrac{1}{x}=\dfrac{1}{z}+\dfrac{1}{y}=\dfrac{1}{x}+\dfrac{1}{z}\\ \Rightarrow\dfrac{1}{x}=\dfrac{1}{y}=\dfrac{1}{z}\\ \Rightarrow x=y=z\)
\(\Rightarrow P=\dfrac{xy+yz+zx}{x^2+y^2+z^2}=\dfrac{x^2+x^2+x^2}{x^2+x^2+x^2}=1\)
W
2
TH
4 tháng 3 2020
\(\left(x-y\right)^2+\left(y-z\right)^2+\left(z-x\right)^2=4\left(x^2+y^2+z^2-xy-xz-yz\right)\)
\(\Leftrightarrow\left(x^2-2xy+y^2\right)+\left(y^2-2zy+z^2\right)+\left(z^2-2xz+x^2\right)=4\left(x^2+y^2+z^2-xy-xz-yz\right)\)
\(\Leftrightarrow2x^2-2xy+2y^2-2yz+2z^2-2xz=4\left(x^2+y^2+z^2-xy-yz-xz\right)\)
\(\Leftrightarrow2\left(x^2+y^2+z^2-xy-yz-zx\right)=4\left(x^2+y^2-xy-xz-yz\right)\)
\(\Leftrightarrow2x^2+2y^2+2z^2-2xy-2yz-2xz=0\)
\(\Leftrightarrow\left(x-y\right)^2+\left(y-z\right)^2+\left(z-x\right)^2=0\)
\(\Leftrightarrow\hept{\begin{cases}x-y=0\\y-z=0\\z-x=0\end{cases}}\Leftrightarrow\hept{\begin{cases}x=y\\y=z\\z=x\end{cases}}\)
\(\Leftrightarrow x=y=z\)
4 tháng 3 2020
\(\left(x-y\right)^2+\left(y-z\right)^2+\left(z-x\right)^2=4.\left(x^2+y^2+z^2-xy-yz-zx\right)\)
\(< =>\left(x^2-2xy+y^2\right)+\left(y^2-2zy+z^2\right)+\left(z^2-2xz+x^2\right)=4.\left(x^2+y^2+z^2-xy-xz-yz\right)\)
\(< =>2x^2-2xy+2y^2-2yz+2z^2-2xz=4.\left(x^2+y^2+z^2-xy-xz-yz\right)\)
\(< =>2.\left(x^2+y^2+x^2-xy-xz-zy\right)=4.\left(x^2+y^2+z^2-xy-xz-zy\right)\)
\(< =>2x^2+2y^2+2z^2-2xy-2xz-2yz=0\)
\(< =>\left(x-y\right)^2+\left(y-z\right)^2+\left(z-x\right)^2=0\)
\(< =>\hept{\begin{cases}x-y=0\\y-z=0\\z-x=0\end{cases}}\)
\(< =>\hept{\begin{cases}x=y\\y=z\\z=x\end{cases}< =>x=y=z}\)
8 tháng 12 2023
Có \(VT=\dfrac{x^2}{x^3-xyz+2013x}+\dfrac{y^2}{y^3-xyz+2013y}+\dfrac{z^2}{z^3-xyz+2013z}\)
\(\ge\dfrac{\left(x+y+z\right)^2}{x^3+y^3+z^3-3xyz+2013\left(x+y+z\right)}\)
\(=\dfrac{\left(x+y+z\right)^2}{\left(x+y+z\right)\left[x^2+y^2+z^2-\left(xy+yz+zx\right)\right]+2013\left(x+y+z\right)}\)
\(=\dfrac{x+y+z}{x^2+y^2+z^2-\left(xy+yz+zx\right)+3\left(xy+yz+zx\right)}\)
(vì \(2013=3.671=3\left(xy+yz+zx\right)\))
\(=\dfrac{x+y+z}{x^2+y^2+z^2+2\left(xy+yz+zx\right)}\)
\(=\dfrac{x+y+z}{\left(x+y+z\right)^2}\)
\(=\dfrac{1}{x+y+z}\)
ĐTXR \(\Leftrightarrow\dfrac{1}{x^2-yz+2013}=\dfrac{1}{y^2-zx+2013}=\dfrac{1}{z^2-xy+2013}\)
\(\Leftrightarrow x^2-yz=y^2-zx=z^2-xy\)
\(\Leftrightarrow x=y=z\) (với \(x,y,z>0\))
Vậy ta có đpcm.
NV
Nguyễn Việt Lâm
Giáo viên
6 tháng 1 2022
\(x^2+y^2+z^2=xy+yz+zx\)
\(\Leftrightarrow2x^2+2y^2+2z^2=2xy+2yz+2zx\)
\(\Leftrightarrow\left(x^2-2xy+y^2\right)+\left(y^2-2yz+z^2\right)+\left(x^2-2zx+z^2\right)=0\)
\(\Leftrightarrow\left(x-y\right)^2+\left(y-z\right)^2+\left(z-x\right)^2=0\)
\(\Leftrightarrow\left\{{}\begin{matrix}x-y=0\\y-z=0\\x-z=0\end{matrix}\right.\) \(\Leftrightarrow x=y=z\)
Mà \(x+y+z=-3\Rightarrow x=y=z=-1\)
\(\Rightarrow x^2+y^3+z^4=\left(-1\right)^2+\left(-1\right)^3+\left(-1\right)^4=1\)
\(x+y+z=0\\ \Rightarrow\left\{{}\begin{matrix}x=-y-z\\y=-z-x\\z=-x-y\end{matrix}\right.\)
\(\dfrac{xy}{x^2+y^2-z^2}+\dfrac{yz}{y^2+z^2-x^2}+\dfrac{zx}{z^2+x^2-y^2}\)
\(=\dfrac{xy}{x^2+y^2-\left(-x-y\right)^2}+\dfrac{yz}{y^2+z^2-\left(-y-z\right)^2}+\dfrac{zx}{z^2+x^2-\left(-z-x\right)^2}\)
\(=\dfrac{xy}{x^2+y^2-\left(x+y\right)^2}+\dfrac{yz}{y^2+z^2-\left(y+z\right)^2}+\dfrac{zx}{z^2+x^2-\left(z+x\right)^2}\)
\(=\dfrac{xy}{x^2+y^2-x^2-2xy-y^2}+\dfrac{yz}{y^2+z^2-y^2-2yz-z^2}+\dfrac{zx}{z^2+x^2-z^2-2zx-x^2}\)
\(=\dfrac{xy}{-2xy}+\dfrac{yz}{-2yz}+\dfrac{zx}{-2zx}\)
\(=-\dfrac{1}{2}-\dfrac{1}{2}-\dfrac{1}{2}\)
\(=-\dfrac{3}{2}\)