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\(T=21x+3y+\dfrac{21}{y}+\dfrac{3}{x}\)
\(T=\dfrac{x}{3}+\dfrac{3}{x}+\dfrac{7y}{3}+\dfrac{21}{y}+\dfrac{62}{3}x+\dfrac{2}{3}y\)
\(T\ge2\sqrt{\dfrac{3x}{3x}}+2\sqrt{\dfrac{147y}{3y}}+\dfrac{62}{3}.3+\dfrac{2}{3}.3=80\)
\(T_{min}=80\) khi \(x=y=3\)
Lời giải:
Áp dụng BĐT Bunhiacopxky:
\(\left(\frac{2}{x}+\frac{8}{9y}+\frac{18}{25z}\right)(x+y+z)\geq (\sqrt{2}+\sqrt{\frac{8}{9}}+\sqrt{\frac{18}{25}})^2\)
$\Leftrightarrow A.2\geq \frac{2312}{225}$
$\Leftrightarrow A\geq \frac{1156}{225}$
Vậy $A_{\min}=\frac{1156}{225}$
Áp dụng bất đẳng thức AM - GM:
\(\dfrac{x^2}{y-1}+4\left(y-1\right)\ge2\sqrt{\dfrac{x^2}{y-1}.4\left(y-1\right)}\)
\(\Rightarrow\dfrac{x^2}{y-1}+4\left(y-1\right)\ge4x\).
Tương tự, \(\dfrac{y^2}{x-1}+4\left(x-1\right)\ge4y\).
Cộng vế với vế hai bđt trên rồi rút gọn ta được:
\(\dfrac{x^2}{y-1}+\dfrac{y^2}{x-1}\ge8\)
\(\Rightarrow P\ge8+2013=2021\).
Đẳng thức xảy ra khi x = y = 2.
Vậy....
\(T\ge\dfrac{\left(x+y+z\right)^2}{x+y+z+\sqrt{xy}+\sqrt{yz}+\sqrt{zx}}\ge\dfrac{\left(x+y+z\right)^2}{x+y+z+x+y+z}=\dfrac{x+y+z}{2}\ge\dfrac{2019}{2}\)
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
\(P=\dfrac{x}{\sqrt{x+y-x}}+\dfrac{y}{\sqrt{x+y-y}}=\dfrac{x}{\sqrt{y}}+\dfrac{y}{\sqrt{x}}\)
\(=\dfrac{x^2}{x\sqrt{y}}+\dfrac{y^2}{y\sqrt{x}}\ge\dfrac{\left(x+y\right)^2}{x\sqrt{y}+y\sqrt{x}}=\dfrac{\left(x+y\right)^2}{\sqrt{xy}\left(\sqrt{x}+\sqrt{y}\right)}\)
\(\ge\dfrac{\left(x+y\right)^2}{\dfrac{x+y}{2}.\left(1.\sqrt{x}+1.\sqrt{y}\right)}\ge\dfrac{\left(x+y\right)^2}{\dfrac{x+y}{2}.\sqrt{\left(1^2+1^2\right)\left(x+y\right)}}=\dfrac{1}{\dfrac{1}{2}\sqrt{2}}=\sqrt{2}\)
"=" khi x = y = 1/2
