Cho x , y , z lớn hơn hoặc bằng 1 . CMR :
\(\dfrac{1}{1+x^2}\) + \(\dfrac{1}{1+y^2}\) \(\ge\) \(\dfrac{2}{1+xy}\)
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Hmm trong đề làm gì có z vậy bạn ?????
\(\Leftrightarrow\left(\dfrac{1}{1+x^2}-\dfrac{1}{1+xy}\right)+\left(\dfrac{1}{1+y^2}-\dfrac{1}{1+xy}\right)\ge0\)
\(\Leftrightarrow\dfrac{1+xy-\left(1+x^2\right)}{\left(1+x^2\right)\left(1+xy\right)}+\dfrac{1+xy-\left(1+y^2\right)}{\left(1+y^2\right)\left(1+xy\right)}\ge0\)
\(\Leftrightarrow\dfrac{-x\left(x-y\right)\left(1+y^2\right)}{\left(1+x^2\right)\left(1+y^2\right)\left(1+xy\right)}+\dfrac{y\left(x-y\right)\left(1+x^2\right)}{\left(1+y^2\right)\left(1+x^2\right)\left(1+xy\right)}\ge0\)
\(\Leftrightarrow\left(x-y\right)\left(-x+y-xy^2+x^2y\right)\ge0\)
\(\Leftrightarrow\left(x-y\right)^2\left(xy-1\right)\ge0\left(\forall x;y\ge0\right)\)
Vậy \(\dfrac{1}{1+x^2}+\dfrac{1}{1+y^2}\ge\dfrac{2}{1+xy}\)
1)\(\Leftrightarrow\left[{}\begin{matrix}\left|x-2\right|+3=5\\\left|x-2\right|+3=-5\end{matrix}\right.\)
\(\Leftrightarrow\left[{}\begin{matrix}\left|x-2\right|=2\\\left|x-2\right|=-8\left(loai\right)\end{matrix}\right.\)
\(\Leftrightarrow\left[{}\begin{matrix}x-2=2\\x-2=-2\end{matrix}\right.\)
\(\Leftrightarrow\left[{}\begin{matrix}x=4\\x=0\end{matrix}\right.\)
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 $x=y=z=1$
\(\left(x^2-y^2\right)^2=\left(x-y\right)^2\left(x+y\right)^2\) \(\Rightarrow\left\{{}\begin{matrix}x;y>0\\x+y< 1\end{matrix}\right.\)=> dccm sai = > người ra đề sai họăc người chép đề sai ;
Ta có: \(\dfrac{1}{1+x^2}+\dfrac{1}{1+y^2}\ge\dfrac{2}{1+xy}\)
\(\Leftrightarrow\left(\dfrac{1}{1+x^2}-\dfrac{1}{1+y^2}\right)+\left(\dfrac{1}{1+y^2}-\dfrac{1}{xy}\right)\ge0\)
\(\Leftrightarrow\dfrac{xy-x^2}{\left(1+x^2\right)\left(1+xy\right)}+\dfrac{xy-y^2}{\left(1+y^2\right)\left(1+xy\right)}\ge0\)
\(\Leftrightarrow\dfrac{x\left(y-x\right)}{\left(1+x^2\right)\left(1+xy\right)}+\dfrac{y\left(x-y\right)}{\left(1+y^2\right)\left(1+xy\right)}\ge0\)
\(\Leftrightarrow\dfrac{\left(y-x\right)^2\left(xy-1\right)}{\left(1+x^2\right)\left(1+y^2\right)\left(1+xy\right)}\ge0\)
BĐT cuối đúng vì x.y > 0 => đpcm
có vẻ viết nhầm Bước2