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Áp dụng bđt Mincopxki \(\sqrt{a^2+b^2}+\sqrt{c^2+d^2}\ge\sqrt{\left(a+c\right)^2+\left(b+d\right)^2}\) ta được
\(VT\ge\sqrt{\left(x+y\right)^2+\left(\frac{1}{x}+\frac{1}{y}\right)^2}+\sqrt{z^2+\frac{1}{z^2}}\)
\(\ge\sqrt{\left(x+y+z\right)^2+\left(\frac{1}{x}+\frac{1}{y}+\frac{1}{z}\right)^2}\)
Áp dụng bđt Cô-si có
\(\left(x+y+z\right)^2+\left(\frac{1}{x}+\frac{1}{y}+\frac{1}{z}\right)^2\ge9\sqrt[3]{\left(xyz\right)^2}+\frac{9}{\sqrt[3]{\left(xyz\right)^2}}\)
Đặt \(\sqrt[3]{\left(xyz\right)^2}=t\)
\(\Rightarrow0\le t=\sqrt[3]{\left(xyz\right)^2}\le\left(\frac{x+y+z}{3}\right)^2=\frac{1}{4}\)
Khi đó \(VT\ge\sqrt{9t+\frac{9}{t}}=\sqrt{3\left(48t+\frac{3}{t}-45t\right)}\ge\sqrt{3\left(2.\sqrt{3.48}-\frac{45}{4}\right)}=\frac{3\sqrt{17}}{2}\)
![](https://rs.olm.vn/images/avt/0.png?1311)
Theo AM-GM: \(x^3+y^2\ge2\sqrt{x^3y^2}=2xy\sqrt{x}\)
\(\Rightarrow\frac{2\sqrt{x}}{x^3+y^2}\le\frac{2\sqrt{x}}{2xy\sqrt{x}}=\frac{1}{xy}\)
Tương tự: \(\frac{2\sqrt{y}}{y^3+z^2}\le\frac{1}{yz}\)
\(\frac{2\sqrt{z}}{z^3+x^2}\le\frac{1}{zx}\)
Cộng vế với vế => \(VT\le\frac{1}{xy}+\frac{1}{yz}+\frac{1}{zx}\)
Theo AM-GM; \(VT\le\frac{1}{xy}+\frac{1}{yz}+\frac{1}{zx}\le\frac{\frac{1}{x^2}+\frac{1}{y^2}+\frac{1}{y^2}+\frac{1}{z^2}+\frac{1}{z^2}+\frac{1}{x^2}}{2}=\frac{1}{x^2}+\frac{1}{y^2}+\frac{1}{z^2}\)
Dấu " = " xảy ra <=> x=y=z=1
Áp dụng bất đẳng thức Cacuhy - Schwarz
\(\Rightarrow\hept{\begin{cases}x^3+y^2\ge2\sqrt{x^3y^2}=2xy\sqrt{x}\\y^3+z^2\ge2\sqrt{y^3z^2}=2yz\sqrt{y}\\z^3+x^2\ge2\sqrt{z^3x^2}=2xz\sqrt{z}\end{cases}}\)
\(\Rightarrow\hept{\begin{cases}\frac{2\sqrt{x}}{x^3+y^2}\le\frac{2\sqrt{x}}{2xy\sqrt{x}}=\frac{1}{xy}\\\frac{2\sqrt{y}}{y^3+z^2}\le\frac{2\sqrt{y}}{2yz\sqrt{y}}=\frac{1}{yz}\\\frac{2\sqrt{z}}{z^3+x^2}\le\frac{2\sqrt{z}}{2xz\sqrt{z}}=\frac{1}{xz}\end{cases}}\)
\(\Rightarrow VT\le\frac{1}{xy}+\frac{1}{yz}+\frac{1}{xz}\left(1\right)\)
Áp dụng bất đẳng thức Cacuchy Schwarz
\(\Rightarrow\hept{\begin{cases}\frac{1}{x^2}+\frac{1}{y^2}\ge2\sqrt{\frac{1}{x^2y^2}}=\frac{2}{xy}\\\frac{1}{y^2}+\frac{1}{z^2}\ge2\sqrt{\frac{1}{y^2z^2}}=\frac{2}{yz}\\\frac{1}{z^2}+\frac{1}{x^2}\ge2\sqrt{\frac{1}{z^2x^2}}=\frac{2}{xz}\end{cases}}\)
\(\Rightarrow2\left(\frac{1}{x^2}+\frac{1}{y^2}+\frac{1}{z^2}\right)\ge2\left(\frac{1}{xy}+\frac{1}{yz}+\frac{1}{xz}\right)\)
\(\Rightarrow\frac{1}{x^2}+\frac{1}{y^2}+\frac{1}{z^2}\ge\frac{1}{xy}+\frac{1}{yz}+\frac{1}{xz}\left(2\right)\)
Từ (1) và (2)
\(\Rightarrow VT\le\frac{1}{x^2}+\frac{1}{y^2}+\frac{1}{z^2}\)
\(\Leftrightarrow\frac{2\sqrt{x}}{x^3+y^2}+\frac{2\sqrt{y}}{y^3+z^2}+\frac{2\sqrt{z}}{z^3+x^2}\le\frac{1}{x^2}+\frac{1}{y^2}+\frac{1}{z^2}\left(đpcm\right)\)
![](https://rs.olm.vn/images/avt/0.png?1311)
\(VT=\Sigma_{cyc}\frac{2\sqrt{x}}{x^3+y^2}\le\Sigma_{cyc}\frac{2\sqrt{x}}{2\sqrt{x^3y^2}}=\Sigma_{cyc}\frac{1}{\sqrt{x^2y^2}}=\Sigma_{cyc}\frac{1}{xy}\)
\(=\frac{1}{xy}+\frac{1}{yz}+\frac{1}{zx}\le\frac{1}{x^2}+\frac{1}{y^2}+\frac{1}{z^2}\) (áp dụng BĐT quen thuộc \(ab+bc+ca\le a^2+b^2+c^2\))
Đẳng thức xảy ra khi x = y = z = 1
Sửa đề : \(\frac{2\sqrt{x}}{x^3+y^2}+\frac{2\sqrt{y}}{y^3+z^2}+\frac{2\sqrt{z}}{z^3+x^2}\)
Áp dụng bất đẳng thức Cauchy - Schwarz
\(\Rightarrow\hept{\begin{cases}x^3+y^2\ge2\sqrt{x^3y^2}=2xy\sqrt{x}\\y^3+z^2\ge2\sqrt{y^3z^2}=2yz\sqrt{y}\\z^3+x^2\ge2\sqrt{z^3x^2}=2xz\sqrt{z}\end{cases}}\)
\(\Rightarrow\hept{\begin{cases}\frac{2\sqrt{x}}{x^3+y^2}\le\frac{2\sqrt{x}}{2xy\sqrt{x}}=\frac{1}{xy}\\\frac{2\sqrt{y}}{y^3+z^2}\le\frac{2\sqrt{y}}{2yz\sqrt{y}}=\frac{1}{yz}\\\frac{2\sqrt{z}}{z^3+x^2}\le\frac{2\sqrt{z}}{2xz\sqrt{z}}=\frac{1}{xz}\end{cases}}\)
\(\Rightarrow VT\le\frac{1}{xy}+\frac{1}{yz}+\frac{1}{xz}\left(1\right)\)
Áp dụng bất đẳng thức Cauchy - Schwarz
\(\Rightarrow\hept{\begin{cases}\frac{1}{x^2}+\frac{1}{y^2}\ge2\sqrt{\frac{1}{x^2y^2}}=\frac{2}{xy}\\\frac{1}{y^2}+\frac{1}{z^2}\ge2\sqrt{\frac{1}{y^2z^2}}=\frac{2}{yz}\\\frac{1}{z^2}+\frac{1}{x^2}\ge2\sqrt{\frac{1}{x^2z^2}}=\frac{2}{xz}\end{cases}}\)
\(\Rightarrow2\left(\frac{1}{x^2}+\frac{1}{y^2}+\frac{1}{z^2}\right)\ge2\left(\frac{1}{xy}+\frac{1}{yz}+\frac{1}{xz}\right)\)
\(\Rightarrow\frac{1}{x^2}+\frac{1}{y^2}+\frac{1}{z^2}\ge\frac{1}{xy}+\frac{1}{yz}+\frac{1}{xz}\left(2\right)\)
Từ (1) và (2) :
\(\Rightarrow VT\le\frac{1}{x^2}+\frac{1}{y^2}+\frac{1}{z^2}\)
\(\Leftrightarrow\frac{2\sqrt{x}}{x^3+y^2}+\frac{2\sqrt{y}}{y^3+z^2}+\frac{2\sqrt{z}}{z^3+x^2}\le\frac{1}{x^2}+\frac{1}{y^2}+\frac{1}{z^2}\left(đpcm\right)\)
Chúc bạn học tốt !!!
![](https://rs.olm.vn/images/avt/0.png?1311)
Bài 2 :
Ta có :
\(2a^2+16ab+7b^2=\left(2a+3b\right)^2-2\left(a-b\right)^2\le\left(2a+3b\right)^2\)
\(\Rightarrow P\ge\frac{25a^2}{2a+3b}+\frac{25b^2}{2b+3c}+\frac{c^2\left(a+3\right)}{a}\)
Áp dụng BĐT Cô - si ta có :
\(\frac{25a^2}{2a+3b}+2a+3b\ge10a\)
\(\frac{25b^2}{2b+3c}+2b+3c\ge10b\)
\(\frac{c^2\left(a+3\right)}{a}=\left(c^2+1\right)+\left(\frac{3c^2}{a}+3a\right)-3a-1\ge2c+6c-3a-1=8c-3a-1\)
Khi đó :
\(P\ge\left(10-2a-3b\right)+\left(10b-2b-3c\right)+\left(8c-3a-1\right)\)
\(\Rightarrow P\ge5\left(a+b+c\right)-1=14\)
Vậy \(MinP=14\) khi a=b=c=1
![](https://rs.olm.vn/images/avt/0.png?1311)
Bạn tham khảo tại đây:
Câu hỏi của Vũ Sơn Tùng - Toán lớp 9 | Học trực tuyến
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CÔSI ta có VT<=1/xy+1/zy+1/zx.
sau đó vẫn áp dụng bất đẳng thức cosi tùng đôi một vế phải đã cho ta sẽ đc điều phải chứng minh
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\(\frac{P}{\sqrt{6}}=\sum\frac{1}{\sqrt{6}}.\frac{1}{\sqrt{2x^2+y^2+3}}\le\frac{1}{2}\sum\left(\frac{1}{6}+\frac{1}{2x^2+y^2+3}\right)\)
\(\frac{P}{\sqrt{6}}\le\frac{1}{4}+\frac{1}{2}\sum\frac{1}{2\left(x^2+1\right)+\left(y^2+1\right)}\le\frac{1}{4}+\frac{1}{2}\sum\frac{1}{4x+2y}\)
\(\frac{P}{\sqrt{6}}\le\frac{1}{4}+\frac{1}{4}\sum\frac{1}{x+x+y}\le\frac{1}{4}+\frac{1}{36}\left(\frac{2}{x}+\frac{1}{y}+\frac{2}{y}+\frac{1}{z}+\frac{2}{z}+\frac{1}{x}\right)\)
\(\frac{P}{\sqrt{6}}\le\frac{1}{4}+\frac{1}{12}\left(\frac{1}{x}+\frac{1}{y}+\frac{1}{z}\right)=\frac{1}{2}\)
\(\Rightarrow P\le\frac{\sqrt{6}}{2}\)
Dấu "=" xảy ra khi \(x=y=z=1\)
\(\sqrt{x^2+\frac{1}{x^2}}+\sqrt{y^2+\frac{1}{y^2}}+\sqrt{z^2+\frac{1}{z^2}}\ge\sqrt{\left(x+y+z\right)^2+\left(\frac{1}{x}+\frac{1}{y}+\frac{1}{z}\right)^2}\)
\(\ge\sqrt{\left(x+y+z\right)^2+\frac{81}{\left(x+y+z\right)^2}}=\sqrt{\left(x+y+z\right)^2+\frac{81}{16\left(x+y+z\right)^2}+\frac{1215}{16\left(x+y+z\right)^2}}\)
\(\ge\sqrt{2\sqrt{\frac{81\left(x+y+z\right)^2}{16\left(x+y+z\right)^2}}+\frac{1215}{16.\left(\frac{3}{2}\right)^2}}=\frac{3\sqrt{17}}{2}\)
Dấu "=" xảy ra khi \(z=y=z=\frac{1}{2}\)