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\(\frac{1}{x}+\frac{1}{y}+\frac{1}{z}=0\Leftrightarrow xy+yz+xz=0\)
\(A=\frac{yz}{x^2+yz+-xy-xz}+\frac{xz}{y^2+zx-xy-yz}+\frac{xy}{z^2+xy-xz-yz}\)
\(A=\frac{yz}{\left(x-y\right)\left(x-z\right)}+\frac{xz}{\left(y-z\right)\left(y-x\right)}+\frac{xy}{\left(z-x\right)\left(z-y\right)}\)
\(A=\frac{yz\left(y-z\right)-xz\left(x-z\right)+xy\left(x-y\right)}{\left(x-z\right)\left(x-y\right)\left(y-z\right)}\)
\(A=\frac{\left(z-x\right)\left(y-z\right)\left(y-x\right)}{\left(x-z\right)\left(x-y\right)\left(y-z\right)}=1\)
2) \(\sum\dfrac{x}{x^2-yz+2013}=\sum\dfrac{x^2}{x^3-xyz+2013x}\ge\dfrac{\left(x+y+z\right)^2}{x^3+y^3+z^3-3xyz+2013\left(x+y+z\right)}=\dfrac{\left(x+y+z\right)^2}{\left(x+y+z\right)^3}=\dfrac{1}{x+y+z}\left(đpcm\right)\)
Ta có BĐT:
\(\frac{1}{xy}+\frac{1}{yz}+\frac{1}{xz}\le\frac{1}{x^2}+\frac{1}{y^2}+\frac{1}{z^2}\)
\(\Leftrightarrow6\left(\frac{1}{xy}+\frac{1}{yz}+\frac{1}{xz}\right)+2016\le6\left(\frac{1}{x^2}+\frac{1}{y^2}+\frac{1}{z^2}\right)+2016\)
\(\Leftrightarrow7.\left(\frac{1}{x^2}+\frac{1}{y^2}+\frac{1}{z^2}\right)\le6\left(\frac{1}{x^2}+\frac{1}{y^2}+\frac{1}{z^2}\right)+2016\)
\(\Leftrightarrow\frac{1}{x^2}+\frac{1}{y^2}+\frac{1}{z^2}\le2016\)
Xét \(P=\frac{1}{\sqrt{3\left(2x^2+y^2\right)}}+\frac{1}{\sqrt{3\left(2y^2+z^2\right)}}+\frac{1}{\sqrt{3\left(2z^2+x^2\right)}}\)
\(P^2=\left(\frac{1}{\sqrt{3}}.\frac{1}{\sqrt{2x^2+y^2}}+\frac{1}{\sqrt{3}}.\frac{1}{\sqrt{2y^2+z^2}}+\frac{1}{\sqrt{3}}.\frac{1}{\sqrt{2z^2+x^2}}\right)^2\)
Áp dụng BĐT Bunhiacopxki ta có:
\(P^2\le\left(\left(\frac{1}{\sqrt{3}}\right)^2+\left(\frac{1}{\sqrt{3}}\right)^2+\left(\frac{1}{\sqrt{3}}\right)^2\right)\left(\left(\frac{1}{\sqrt{2x^2+y^2}}\right)^2+\left(\frac{1}{\sqrt{2y^2+z^2}}\right)^2+\left(\frac{1}{\sqrt{2z^2+x^2}}\right)^2\right)\)
\(\Leftrightarrow P^2\le\frac{1}{2x^2+y^2}+\frac{1}{2y^2+z^2}+\frac{1}{2z^2+x^2}\)
Mặt khác ta có:
\(\frac{1}{2x^2+y^2}=\frac{1}{x^2+x^2+y^2}\le\frac{1}{9}\left(\frac{1}{x^2}+\frac{1}{x^2}+\frac{1}{y^2}\right)\)
\(\frac{1}{2y^2+z^2}\le\frac{1}{9}\left(\frac{1}{y^2}+\frac{1}{y^2}+\frac{1}{z^2}\right)\)
\(\frac{1}{2z^2+x^2}\le\frac{1}{9}\left(\frac{1}{z^2}+\frac{1}{z^2}+\frac{1}{x^2}\right)\)
\(\Rightarrow P^2\le\frac{1}{3}\left(\frac{1}{x^2}+\frac{1}{y^2}+\frac{1}{z^2}\right)\le\frac{1}{3}.2016=672\)
\(\Rightarrow P\le4\sqrt{42}\)
Dấu '=' xảy ra khi \(x=y=z=\sqrt{\frac{1}{672}}\)
Ta có \(\dfrac{x}{1+y^2}=x-\dfrac{xy^2}{1+y^2}\ge x-\dfrac{xy}{2}\)
Tương tự ta có \(\Sigma\left(\dfrac{x}{1+y^2}\right)\ge\Sigma\left(x-\dfrac{xy}{2}\right)=3-\left(\dfrac{xy+yz+xz}{2}\right)\)
Theo hệ quả của bđt Cauchy ta có \(\left(x+y+z\right)^2\ge3\left(xy+yz+xz\right)\)
\(\Leftrightarrow3\ge xy+yz+xz\Rightarrow3-\left(\dfrac{xy+yz+xz}{2}\right)\ge3-\dfrac{3}{2}=\dfrac{3}{2}\)
\(\Rightarrow\dfrac{x}{1+y^2}+\dfrac{y}{1+z^2}+\dfrac{z}{1+x^2}\ge\dfrac{3}{2}\) ( 1 )
Ta lại có \(\dfrac{1}{1+y^2}=1-\dfrac{y^2}{1+y^2}\ge1-\dfrac{y}{2}\)
Tương tự ta có \(\Sigma\left(\dfrac{1}{1+y^2}\right)\ge\Sigma\left(1-\dfrac{y}{2}\right)=3-\left(\dfrac{x+y+z}{2}\right)=3-\dfrac{3}{2}=\dfrac{3}{2}\)
\(\Rightarrow\dfrac{1}{1+y^2}+\dfrac{1}{1+z^2}+\dfrac{1}{1+x^2}\ge\dfrac{3}{2}\) ( 2 )
Từ ( 1 ) và ( 2 ) \(\Rightarrow Q\ge\dfrac{3}{2}+\dfrac{3}{2}=3\)
Vậy \(Q_{min}=3\)
Dấu '' = '' xảy ra khi \(x=y=z=1\)
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Lời giải:
\(P=\sum \frac{1}{2xy^2+1}=\sum (1-\frac{2xy^2}{2xy^2+1})\)
\(=3-2\sum\frac{xy^2}{2xy^2+1}\geq 3-2\sum \frac{xy^2}{3\sqrt[3]{x^2y^4}}\) theo BĐT AM-GM.
\(=3-\frac{2}{3}\sum \sqrt[3]{xy^2}\)
Tiếp tục áp dụng BĐT AM-GM:
\(\sqrt[3]{xy^2}\leq \frac{x+y+y}{3}\Rightarrow \sum \sqrt[3]{xy^2}\leq \frac{3(x+y+z)}{3}=3\)
$\Rightarrow P\geq 3-\frac{2}{3}.3=1$
Vậy $P_{\min}=1$. Giá trị này đạt tại $x=y=z=1$