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![](https://rs.olm.vn/images/avt/0.png?1311)
\(\sqrt{\dfrac{x^3}{y^3}}+\sqrt{\dfrac{x^3}{y^3}}+1\ge\dfrac{3x}{y}\) ; \(2\sqrt{\dfrac{y^3}{z^3}}+1\ge\dfrac{3y}{z}\) ; \(2\sqrt{\dfrac{z^3}{x^3}}+1\ge\dfrac{3z}{x}\)
\(\Rightarrow2VT+3\ge\dfrac{3x}{y}+\dfrac{3y}{z}+\dfrac{3z}{x}\)
\(\Rightarrow2VT+3\ge\dfrac{2x}{y}+\dfrac{2y}{z}+\dfrac{2z}{x}+3\sqrt[3]{\dfrac{xyz}{xyz}}\)
\(\Rightarrow VT\ge\dfrac{x}{y}+\dfrac{y}{z}+\dfrac{z}{x}\) (đpcm)
![](https://rs.olm.vn/images/avt/0.png?1311)
\(\Rightarrow\left(x+y+z\right)^2\ge\left(\dfrac{1}{x}+\dfrac{1}{y}+\dfrac{1}{z}\right)^2\ge3\left(\dfrac{1}{xy}+\dfrac{1}{yz}+\dfrac{1}{xz}\right)=\dfrac{3\left(x+y+z\right)}{xyz}\Rightarrow x+y+z\ge\dfrac{3}{xyz}\)
\(x+y+z=\dfrac{x+y+z}{3}+\dfrac{2\left(x+y+z\right)}{3}\ge\dfrac{1}{3}\left(\dfrac{1}{x}+\dfrac{1}{y}+\dfrac{1}{z}\right)+\dfrac{2}{3}.\dfrac{3}{xyz}\ge\dfrac{1}{3}\left(\dfrac{9}{x+y+z}\right)+\dfrac{2}{xyz}=\dfrac{3}{x+y+z}+\dfrac{2}{xyz}\left(đpcm\right)\)
\(dấu"="xảy\) \(ra\Leftrightarrow x=y=z=1\)
![](https://rs.olm.vn/images/avt/0.png?1311)
Lời giải:
Ta có: \(xy+yz+xz=3xyz\Rightarrow \frac{1}{x}+\frac{1}{y}+\frac{1}{z}=3\)
Mà theo BĐT Cauchy-Schwarz: \(\frac{1}{x}+\frac{1}{y}+\frac{1}{z}\geq \frac{9}{x+y+z}\)
Do đó: \(3\geq \frac{9}{x+y+z}\Rightarrow x+y+z\geq 3\)
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Ta có: \(\text{VT}=x-\frac{xz}{x^2+z}+y-\frac{xy}{y^2+x}+z-\frac{yz}{z^2+y}\)
\(=(x+y+z)-\left(\frac{xy}{y^2+x}+\frac{yz}{z^2+y}+\frac{xz}{x^2+z}\right)\)
\(\geq x+y+z-\frac{1}{2}\left(\frac{xy}{\sqrt{xy^2}}+\frac{yz}{\sqrt{z^2y}}+\frac{xz}{\sqrt{x^2z}}\right)\) (AM-GM)
\(=x+y+z-\frac{1}{2}(\sqrt{x}+\sqrt{y}+\sqrt{z})\)
Tiếp tục AM-GM: \(\sqrt{x}+\sqrt{y}+\sqrt{z}\leq \frac{x+1}{2}+\frac{y+1}{2}+\frac{z+1}{2}=\frac{x+y+z+3}{2}\)
Suy ra:
\(\text{VT}\geq x+y+z-\frac{1}{2}.\frac{x+y+z+3}{2}=\frac{3}{4}(x+y+z)-\frac{3}{4}\)
\(\geq \frac{9}{4}-\frac{3}{4}=\frac{3}{2}=\frac{1}{2}\left(\frac{1}{x}+\frac{1}{y}+\frac{1}{z}\right)\)
Ta có đpcm
Dấu bằng xảy ra khi $x=y=z=1$
![](https://rs.olm.vn/images/avt/0.png?1311)
\(x+y+z=xyz\Rightarrow\dfrac{1}{xy}+\dfrac{1}{yz}+\dfrac{1}{zx}=1\)
Đặt \(\left(a;b;c\right)=\left(\dfrac{1}{x};\dfrac{1}{y};\dfrac{1}{z}\right)\Rightarrow ab+bc+ca=1\)
Đặt vế trái là P, ta có:
\(P=\dfrac{a}{\sqrt{a^2+1}}+\dfrac{b}{\sqrt{b^2+1}}+\dfrac{c}{\sqrt{c^2+1}}\)
\(P=\dfrac{a}{\sqrt{a^2+ab+bc+ca}}+\dfrac{b}{\sqrt{b^2+ab+bc+ca}}+\dfrac{c}{\sqrt{c^2+ab+bc+ca}}\)
\(P=\dfrac{a}{\sqrt{\left(a+b\right)\left(a+c\right)}}+\dfrac{b}{\sqrt{\left(a+b\right)\left(b+c\right)}}+\dfrac{c}{\sqrt{\left(a+c\right)\left(b+c\right)}}\)
\(P\le\dfrac{1}{2}\left(\dfrac{a}{a+b}+\dfrac{a}{a+c}\right)+\dfrac{1}{2}\left(\dfrac{b}{a+b}+\dfrac{b}{b+c}\right)+\dfrac{1}{2}\left(\dfrac{c}{a+c}+\dfrac{c}{b+c}\right)=\dfrac{3}{2}\)
Dấu "=" xảy ra khi \(a=b=c=\dfrac{1}{\sqrt{3}}\) hay \(x=y=z=\sqrt{3}\)
![](https://rs.olm.vn/images/avt/0.png?1311)
Đặt vế trái là P, ta có:
\(P\le\sqrt{3\left(\dfrac{x}{z+3x}+\dfrac{y}{x+3y}+\dfrac{z}{y+3z}\right)}\)
Nên ta chỉ cần chứng mình: \(\sqrt{3\left(\dfrac{x}{z+3x}+\dfrac{y}{x+3y}+\dfrac{z}{y+3z}\right)}\le\dfrac{3}{2}\)
\(\Leftrightarrow\dfrac{x}{z+3x}+\dfrac{y}{x+3y}+\dfrac{z}{y+3z}\le\dfrac{3}{4}\)
\(\Leftrightarrow\dfrac{3x}{z+3x}-1+\dfrac{3y}{x+3y}-\dfrac{3z}{y+3z}-1\le\dfrac{9}{4}-3\)
\(\Leftrightarrow\dfrac{z}{z+3x}+\dfrac{x}{x+3y}+\dfrac{y}{y+3z}\ge\dfrac{3}{4}\)
BĐT trên đúng do:
\(\dfrac{z}{z+3x}+\dfrac{x}{x+3y}+\dfrac{y}{y+3z}=\dfrac{z^2}{z^2+3zx}+\dfrac{x^2}{x^2+3xy}+\dfrac{y^2}{y^2+3yz}\)
\(\ge\dfrac{\left(x+y+z\right)^2}{\left(x+y+z\right)^2+xy+yz+zx}\ge\dfrac{\left(x+y+z\right)^2}{\left(x+y+z\right)^2+\dfrac{1}{3}\left(x+y+z\right)^2}=\dfrac{3}{4}\)
Theo de bai ta co: \(x=\dfrac{y^2}{z}\Rightarrow\dfrac{z}{x}=\dfrac{z^2}{y^2}\left(1\right)\)
Va \(y=\dfrac{z^2}{x}\left(2\right)\)
Tu (1),(2) suy ra y=z \(\Rightarrow x=y=z\)
suy ra A=1