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\(T\ge\dfrac{\left(x+y+z\right)^2}{x+y+z+\sqrt{xy}+\sqrt{yz}+\sqrt{zx}}\ge\dfrac{\left(x+y+z\right)^2}{x+y+z+x+y+z}=\dfrac{x+y+z}{2}\ge\dfrac{2019}{2}\)
áp dụng BĐT:\(\dfrac{a^2}{x}+\dfrac{b^2}{y}+\dfrac{c^2}{z}\) với a,b,c,x,y,z là số dương
ta có BĐT Bunhiacopxki cho 3 bộ số:\(\left(\dfrac{a}{\sqrt{x}};\sqrt{x}\right);\left(\dfrac{b}{\sqrt{y}};\sqrt{y}\right);\left(\dfrac{c}{\sqrt{z}};\sqrt{z}\right)\)
ta có :
\(\dfrac{a^2}{x}+\dfrac{b^2}{y}+\dfrac{c^2}{z}\left(x+y+z\right)\)\(=\left[\left(\dfrac{a}{\sqrt{x}}\right)^2+\left(\dfrac{b}{\sqrt{y}}\right)^2+\left(\dfrac{c}{\sqrt{z}}\right)^2\right]\).\(\left[\left(\sqrt{x}\right)^2+\left(\sqrt{y}\right)^2+\left(\sqrt{z}\right)^2\right]\)\(\ge\left(\dfrac{a}{\sqrt{x}}.\sqrt{x}+\dfrac{b}{\sqrt{y}}.\sqrt{y}+\dfrac{c}{\sqrt{z}}.\sqrt{z}\right)^2=\left(a+b+c\right)^2\)
lúc đó ta có :\(\dfrac{a^2}{x}+\dfrac{b^2}{y}+\dfrac{c^2}{z}\ge\dfrac{\left(a+b+c\right)^2}{x+y+z}\)
ta có \(T=\dfrac{x^2}{x+\sqrt{yz}}+\dfrac{y^2}{y+\sqrt{zx}}+\dfrac{z^2}{z+\sqrt{xy}}\)\(\ge\dfrac{\left(x+y+z\right)^2}{x+\sqrt{yz}+y+\sqrt{zx}+z+\sqrt{xy}}\) mà ta có :
\(\sqrt{yz}+\sqrt{zx}+\sqrt{xy}\)\(\le\dfrac{x+y}{2}+\dfrac{x+z}{2}+\dfrac{z+y}{2}\)\(\Rightarrow\sqrt{yz}+\sqrt{zx}+\sqrt{xy}\le x+y+z\)
\(\Rightarrow T=\dfrac{2019}{2}\Leftrightarrow x=y=z=673\)
vậy \(\text{MinT}=\dfrac{2019}{2}\) khi và chỉ khi x=y=z=673
\(3=x+y+xy\le\sqrt{2\left(x^2+y^2\right)}+\dfrac{x^2+y^2}{2}\)
\(\Rightarrow\left(\sqrt{x^2+y^2}-\sqrt{2}\right)\left(\sqrt{x^2+y^2}+3\sqrt{2}\right)\ge0\)
\(\Rightarrow x^2+y^2\ge2\)
\(\Rightarrow-\left(x^2+y^2\right)\le-2\)
\(P=\sqrt{9-x^2}+\sqrt{9-y^2}+\dfrac{x+y}{4}\le\sqrt{2\left(9-x^2+9-y^2\right)}+\dfrac{\sqrt{2\left(x^2+y^2\right)}}{4}\)
\(P\le\sqrt{2\left(18-x^2-y^2\right)}+\dfrac{1}{4}.\sqrt{2\left(x^2+y^2\right)}\)
\(P\le\left(\sqrt{2}-1\right)\sqrt{18-x^2-y^2}+\sqrt[]{2}\sqrt{\dfrac{\left(18-x^2-y^2\right)}{2}}+\dfrac{1}{2}\sqrt{\dfrac{x^2+y^2}{2}}\)
\(P\le\left(\sqrt{2}-1\right).\sqrt{18-2}+\sqrt{\left(2+\dfrac{1}{4}\right)\left(\dfrac{18-x^2-y^2+x^2+y^2}{2}\right)}=\dfrac{1+8\sqrt{2}}{2}\)
Dấu "=" xảy ra khi \(x=y=1\)
\(x\ge xy+1\Rightarrow1\ge y+\dfrac{1}{x}\ge2\sqrt{\dfrac{y}{x}}\Rightarrow\dfrac{y}{x}\le\dfrac{1}{4}\)
\(Q^2=\dfrac{x^2+2xy+y^2}{3x^2-xy+y^2}=\dfrac{\left(\dfrac{y}{x}\right)^2+2\left(\dfrac{y}{x}\right)+1}{\left(\dfrac{y}{x}\right)^2-\dfrac{y}{x}+3}\)
Đặt \(\dfrac{y}{x}=t\le\dfrac{1}{4}\)
\(Q^2=\dfrac{t^2+2t+1}{t^2-t+3}=\dfrac{t^2+2t+1}{t^2-t+3}-\dfrac{5}{9}+\dfrac{5}{9}\)
\(Q^2=\dfrac{\left(4t-1\right)\left(t+6\right)}{9\left(t^2-t+3\right)}+\dfrac{5}{9}\le\dfrac{5}{9}\)
\(\Rightarrow Q_{max}=\dfrac{\sqrt{5}}{3}\) khi \(t=\dfrac{1}{4}\) hay \(\left(x;y\right)=\left(2;\dfrac{1}{2}\right)\)
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\(P=\dfrac{x^2+y^2+6}{x+y}=\dfrac{x^2+y^2+2xy+4}{x+y}=\dfrac{\left(x+y\right)^2+4}{x+y}=x+y+\dfrac{4}{x+y}\)
\(P\ge2\sqrt{\left(x+y\right).\dfrac{4}{x+y}}=4\)
\(P_{min}=4\) khi \(x=y=1\)
\(P\le\frac{x}{2\sqrt{x^4.y^2}}+\frac{y}{2\sqrt{x^2.y^4}}=\frac{x}{2x^2y}+\frac{y}{2xy^2}=\frac{1}{2xy}+\frac{1}{2xy}=\frac{1}{xy}=1\)
Dấu "=" xảy ra khi x=y=1
\(1+x+y=\sqrt{x}+\sqrt{xy}+\sqrt{y}\)
\(\Leftrightarrow2\left(1+x+y\right)=2\left(\sqrt{x}+\sqrt{xy}+\sqrt{y}\right)\)
\(\Leftrightarrow2+2x+2y=2\sqrt{x}+2\sqrt{xy}+2\sqrt{y}\)
\(\Leftrightarrow2x+2y+2-2\sqrt{x}-2\sqrt{xy}-2\sqrt{y}=0\)
\(\Leftrightarrow\left(x-2\sqrt{xy}+y\right)+\left(x-2\sqrt{x}+1\right)+\left(y-2\sqrt{y}+1\right)=0\)
\(\Leftrightarrow\left(\sqrt{x}-\sqrt{y}\right)^2+\left(\sqrt{x}-1\right)^2+\left(\sqrt{y}-1\right)^2=0\)
\(\Leftrightarrow\hept{\begin{cases}\sqrt{x}=\sqrt{y}\\\sqrt{x}=1\\\sqrt{y}=1\end{cases}}\)
\(\Leftrightarrow x=y=1\)
\(\Rightarrow S=x^{2013}+y^{2013}=1+1=2\)
Lời giải:
Áp dụng BĐT Cô-si và Cauchy-Schwarz cho các số dương ta có:
$A=\frac{1}{x}+\frac{1}{\sqrt{xy}}\geq \frac{1}{x}+\frac{1}{\frac{x+y}{2}}=\frac{1}{x}+\frac{2}{x+y}=2(\frac{1}{2x}+\frac{1}{x+y})$
$\geq 2.\frac{4}{2x+x+y}=\frac{8}{3x+y}\geq \frac{8}{4}=2$
Vậy $A_{\min}=2$. Giá trị này đạt được tại $x=y; 3x+y=4\Leftrightarrow x=y=1$
\(\left(x+9\right)\left(y+11\right)=12\sqrt{xy}\) hay \(\left(x+9\right)\left(y+1\right)=12\sqrt{xy}\) bạn?
(x+9)(y+11)=12√xy nha bạn