cho 3 so thoa man \(\dfrac{1}{x}+\dfrac{1}{y}+\dfrac{1}{z}=0\)
CM: \(\dfrac{1}{x^2+2yz}+\dfrac{1}{y^2+2zx}+\dfrac{1}{z^2+2xy}=0\)
K
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UK
13 tháng 12 2017
Áp dụng BĐT Cauchy-Schwarz, ta có:
\(VT\ge\dfrac{\left(1+1+1\right)^2}{x^2+y^2+z^2+2xy+2yz+2xz}=\dfrac{9}{\left(x+y+z\right)^2}=9\)
Đẳng thức xảy ra khi \(x=y=z=\dfrac{1}{3}\)
H
21 tháng 12 2019
\(\frac{1}{x}+\frac{1}{y}+\frac{1}{z}=0\Leftrightarrow xy+yz+xz=0\)
\(A=\frac{yz}{x^2+yz+-xy-xz}+\frac{xz}{y^2+zx-xy-yz}+\frac{xy}{z^2+xy-xz-yz}\)
\(A=\frac{yz}{\left(x-y\right)\left(x-z\right)}+\frac{xz}{\left(y-z\right)\left(y-x\right)}+\frac{xy}{\left(z-x\right)\left(z-y\right)}\)
\(A=\frac{yz\left(y-z\right)-xz\left(x-z\right)+xy\left(x-y\right)}{\left(x-z\right)\left(x-y\right)\left(y-z\right)}\)
\(A=\frac{\left(z-x\right)\left(y-z\right)\left(y-x\right)}{\left(x-z\right)\left(x-y\right)\left(y-z\right)}=1\)
6 tháng 7 2018
\(\text{a) }\sqrt{\dfrac{1}{x^2}+\dfrac{1}{y^2}+\dfrac{1}{z^2}}\\ =\sqrt{\dfrac{1}{x^2}+\dfrac{1}{y^2}+\dfrac{1}{z^2}+2\left(\dfrac{1}{xy}+\dfrac{1}{xz}+\dfrac{1}{yz}\right)-2\left(\dfrac{1}{xy}+\dfrac{1}{xz}+\dfrac{1}{yz}\right)}\\ =\sqrt{\left(\dfrac{1}{x}+\dfrac{1}{y}+\dfrac{1}{z}\right)^2-2\cdot\dfrac{x+y+z}{xyz}}\\ =\left|\dfrac{1}{x}+\dfrac{1}{y}+\dfrac{1}{z}\right|\)
\(\text{b) }\sqrt{1+\dfrac{1}{2^2}+\dfrac{1}{3^2}}+\sqrt{1+\dfrac{1}{3^2}+\dfrac{1}{4^2}}+...+\sqrt{1+\dfrac{1}{2017^2}+\dfrac{1}{2018^2}}\\ =1+\dfrac{1}{2}-\dfrac{1}{3}+1+\dfrac{1}{3}-\dfrac{1}{4}+...+1+\dfrac{1}{2017}-\dfrac{1}{2018}\\ =2016+\dfrac{1}{2}-\dfrac{1}{2018}\\ =\dfrac{2034698}{1009}\)
ĐK: \(x,y,z\ne0\)
\(\dfrac{1}{x}+\dfrac{1}{y}+\dfrac{1}{z}=0\Leftrightarrow xyz\left(\dfrac{1}{x}+\dfrac{1}{y}+\dfrac{1}{z}\right)=0\Leftrightarrow xy+xz+yz=0\)
\(\Rightarrow\left\{{}\begin{matrix}xy=-xz-yz\\xz--xy-yz\\yz=-xy-xz\end{matrix}\right.\)
Ta có:
\(x^2+2yz=x^2+yz+yz=x^2+yz-xy-xz=x\left(x-y\right)-z\left(x-y\right)\)
\(=\left(x-y\right)\left(x-z\right)\Rightarrow\dfrac{1}{x^2+2yz}=\dfrac{1}{\left(x-y\right)\left(x-z\right)}\)
Tương tự: \(\dfrac{1}{y^2+2xz}=\dfrac{1}{\left(y-x\right)\left(y-z\right)}=\dfrac{-1}{\left(x-y\right)\left(y-z\right)}\)
\(\dfrac{1}{z^2+2xy}=\dfrac{1}{\left(z-x\right)\left(z-y\right)}=\dfrac{1}{\left(x-z\right)\left(y-z\right)}\)
Cộng vế với vế ta được:
\(\dfrac{1}{x^2+2yz}+\dfrac{1}{y^2+2xz}+\dfrac{1}{z^2+2xy}=\dfrac{1}{\left(x-y\right)\left(x-z\right)}+\dfrac{-1}{\left(x-y\right)\left(y-z\right)}+\dfrac{1}{\left(x-z\right)\left(y-z\right)}\)
\(=\dfrac{y-z-\left(x-z\right)+x-y}{\left(x-y\right)\left(x-z\right)\left(y-z\right)}=\dfrac{y-z-x+z+x-y}{\left(x-y\right)\left(x-z\right)\left(y-z\right)}=0\) (đpcm)