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\(A=\frac{1}{\sqrt{x^2-xy+y^2}}+\frac{1}{\sqrt{y^2-yz+z^2}}+\frac{1}{\sqrt{z^2-zx+x^2}}\)
\(=\frac{1}{\sqrt{\frac{1}{2}\left(x-y\right)^2+\frac{1}{2}\left(x^2+y^2\right)}}+\frac{1}{\sqrt{\frac{1}{2}\left(y-z\right)^2+\frac{1}{2}\left(y^2+z^2\right)}}+\frac{1}{\sqrt{\frac{1}{2}\left(z-x\right)^2+\frac{1}{2}\left(z^2+x^2\right)}}\)
\(\le\frac{1}{\sqrt{\frac{1}{2}\left(x^2+y^2\right)}}+\frac{1}{\sqrt{\frac{1}{2}\left(y^2+z^2\right)}}+\frac{1}{\sqrt{\frac{1}{2}\left(z^2+x^2\right)}}\)
\(\le\frac{2}{x+y}+\frac{2}{y+z}+\frac{2}{z+x}\le\frac{1}{x}+\frac{1}{y}+\frac{1}{z}=1\)
Ta có \(\frac{\sqrt{x^2+2y^2}}{xy}=\sqrt{\frac{1}{y^2}+\frac{2}{x^2}}\)
Áp dụng BĐT Buniacoxki ta có
\(\sqrt{\left(\frac{1}{y^2}+\frac{2}{x^2}\right)\left(1+2\right)}\ge\sqrt{\left(\frac{1}{y}+\frac{2}{x}\right)^2}=\frac{1}{y}+\frac{2}{x}\)
=> \(\sqrt{3}A\ge3\left(\frac{1}{x}+\frac{1}{y}+\frac{1}{z}\right)=3\)
=> \(A\ge\sqrt{3}\)
\(MinA=\sqrt{3}\)khi x=y=z=3
Đặt \(\left(x;y;z\right)=\left(a^3;b^3;c^3\right)\Rightarrow abc=1\)
\(VT=\sum\frac{\sqrt{1+a^6+b^6}}{a^3b^3}\ge\sum\frac{\sqrt{3\sqrt[3]{a^6b^6}}}{a^3b^3}=\sqrt{3}\left(\frac{1}{a^2b^2}+\frac{1}{b^2c^2}+\frac{1}{c^2a^2}\right)\)
\(VT\ge\sqrt{3}.3\sqrt[3]{\frac{1}{a^2b^2.b^2c^2.c^2a^2}}=3\sqrt{3}\)
Dấu "=" xảy ra khi \(a=b=c=1\) hay \(x=y=z=1\)
gọi P là cái 1/x+1/y+1/z nha
1) (1/x+1/y+1/z)^2 = 1/x^2 + 1/y^2 + 1/z^2 + 2/(xy) + 2/(yz) + 2/(zx)
---> 3 = P + 2(x+y+z)/(xyz) = P + 2 ---> P = 1
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Giải
Dự đoán dấu "=" xảy ra khi \(x=y=z=\frac{1}{2}\) khi đó \(P=\frac{3\sqrt{3}}{4}\)
Ta sẽ chứng minh nó là GTNN của \(P\)
Ta có: \(x^2+xy+y^2=\frac{3\left(x+y\right)^2+\left(x-y\right)^2}{4}\ge\frac{3\left(x+y\right)^2}{4}\)
Do đó ta cần chứng minh
\(\frac{x+y}{4yz+1}+\frac{y+z}{4xz+1}+\frac{x+z}{4xy+1}\ge\frac{3}{2}\)
\(\Leftrightarrow\frac{x+y}{\left(y+z\right)^2+1}+\frac{y+z}{\left(x+z\right)^2+1}+\frac{x+z}{\left(x+y\right)^2+1}\ge\frac{3}{2}\)
Ta có: \(x+y+z=\frac{3}{2}\Rightarrow2x+2y+2z=3\)
\(\Rightarrow\left(x+y\right)+\left(y+z\right)+\left(x+z\right)=2\)
Đặt \(\hept{\begin{cases}a=x+y\\b=y+z\\c=z+x\end{cases}}\) thì ta cần chứng minh
\(\frac{a}{b^2+1}+\frac{b}{c^2+1}+\frac{c}{a^2+1}\ge\frac{3}{2}\)\(\forall\hept{\begin{cases}a,b,c>0\\a+b+c=3\end{cases}}\)
Lại có: \(\frac{a}{b^2+1}=a-\frac{ab^2}{b^2+1}\ge a-\frac{ab}{2}\)
Tương tự ta cũng có: \(\frac{b}{c^2+1}\ge b-\frac{bc}{2};\frac{c}{a^2+1}\ge c-\frac{ac}{2}\)
Cộng theo vế các BĐT ta có: \(\frac{a}{b^2+1}+\frac{b}{c^2+1}+\frac{c}{a^2+1}\ge a-\frac{ab}{2}+b-\frac{bc}{2}+c-\frac{ac}{2}\)
\(=\left(a+b+c\right)-\frac{ab+bc+ca}{2}\ge3-\frac{3}{2}=\frac{3}{2}\)
BĐT đã được c/m vậy ta có \(P\ge\frac{3\sqrt{3}}{4}\Leftrightarrow x=y=z=\frac{1}{2}\)
Bài 1:Áp dụng C-S dạng engel
\(\frac{3}{xy+yz+xz}+\frac{2}{x^2+y^2+z^2}=\frac{6}{2\left(xy+yz+xz\right)}+\frac{2}{x^2+y^2+z^2}\)
\(\ge\frac{\left(\sqrt{6}+\sqrt{2}\right)^2}{\left(x+y+z\right)^2}=\left(\sqrt{6}+\sqrt{2}\right)^2>14\)
\(P=\frac{\sqrt{1+x^2+y^2}}{xy}+\frac{\sqrt{1+y^2+z^2}}{yz}+\frac{\sqrt{1+z^2+x^2}}{zx}\)
\(\ge\text{Σ}\frac{\sqrt{\frac{\left(1+x+y\right)^2}{3}}}{xy}\text{=}\frac{1+x+y}{xy\sqrt{3}}\)
\(=\frac{\sqrt{3}}{3}\left(\frac{1+x+y}{xy}+\frac{1+y+z}{yz}+\frac{1+z+x}{zx}\right)\)
\(=\frac{\sqrt{3}}{3}\left(\frac{1}{xy}+\frac{1}{yz}+\frac{1}{xz}+\frac{1}{x}+\frac{1}{y}+\frac{1}{y}+\frac{1}{z}+\frac{1}{z}+\frac{1}{x}\right)\)
\(=\frac{\sqrt{3}}{3}\left(x+y+z+2xy+2yz+2zx\right)\)\(\ge\frac{\sqrt{3}}{3}\left(3\sqrt[3]{xyz}+2\cdot3\sqrt[3]{x^2y^2z^2}\right)=\frac{\sqrt{3}}{3}\left(3+6\right)=3\sqrt{3}\)
Dấu = xảy ra khi \(x=y=z=1\)