Cho a,b,c >0 t/m \(a+b+c\ge3\). Tìm Min của \(S=\frac{a}{\sqrt{b}}+\frac{b}{\sqrt{c}}+\frac{c}{\sqrt{a}}\)
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\(A=\frac{a\sqrt{a}}{\sqrt{a+b+2c}}+\frac{b\sqrt{b}}{\sqrt{b+c+2a}}+\frac{c\sqrt{c}}{\sqrt{c+a+2b}}\)
\(A=\frac{a^2}{\sqrt{a\left(a+b+2c\right)}}+\frac{b^2}{\sqrt{b\left(b+c+2a\right)}}+\frac{c^2}{\sqrt{c\left(c+a+2b\right)}}\)
\(\ge\frac{\left(a+b+c\right)^2}{\sqrt{a\left(a+b+2c\right)}+\sqrt{b\left(b+c+2a\right)}+\sqrt{c\left(c+a+2b\right)}}\)
Xét: \(2\left(\sqrt{a\left(a+b+2c\right)}+\sqrt{b\left(b+c+2a\right)}+\sqrt{c\left(c+a+2b\right)}\right)\)
\(=\sqrt{4a\left(a+b+2c\right)}+\sqrt{4b\left(b+c+2a\right)}+\sqrt{4c\left(c+a+2b\right)}\)
\(\le\frac{4a+a+b+2c+4b+b+c+2a+4c+c+a+2b}{2}=4\left(a+b+c\right)\)
\(\Rightarrow\sqrt{a\left(a+b+2c\right)}+\sqrt{b\left(b+c+2a\right)}+\sqrt{c\left(c+a+2b\right)}\le2\left(a+b+c\right)\)
\(\Rightarrow\frac{\left(a+b+c\right)^2}{\sqrt{a\left(a+b+2c\right)}+\sqrt{b\left(b+c+2a\right)}+\sqrt{c\left(c+a+2b\right)}}\ge\frac{\left(a+b+c\right)^2}{2\left(a+b+c\right)}=\frac{3}{2}\)
\("="\Leftrightarrow a=b=c=1\)
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Đề bài có vấn đề nho nhỏ, thay điểm rơi vào thì vế phải thừa bình phương trong ngoặc
Áp dụng Holder:
\(\left(a^2+\frac{1}{b^2}\right)\left(4+\frac{1}{4}\right)\left(4+\frac{1}{4}\right)\ge\left(\sqrt[3]{16a^2}+\sqrt[3]{\frac{1}{16b^2}}\right)^3\)
\(\Rightarrow\sqrt[3]{17^2\left(a^2+\frac{1}{b^2}\right)}\ge4\sqrt[3]{4a^2}+\frac{1}{\sqrt[3]{b^2}}\)
\(\Rightarrow P=\sqrt[3]{17^2}.S\ge4\sqrt[3]{4}\left(\sqrt[3]{a^2}+\sqrt[3]{b^2}+\sqrt[3]{c^2}\right)+\frac{1}{\sqrt[3]{a^2}}+\frac{1}{\sqrt[3]{b^2}}+\frac{1}{\sqrt[3]{c^2}}\)
\(P=\frac{15}{\sqrt[3]{16}}\sum\sqrt[3]{a^2}+\sum\left(\sqrt[3]{\frac{a^2}{16}}+\frac{1}{\sqrt[3]{a^2}}\right)\)
Ta có: \(3\sqrt[3]{a^2}+\sqrt[3]{4}\ge4\sqrt[12]{4a^6}=4\sqrt[6]{2}.\sqrt{a}\)
Tương tự và cộng lại:
\(\Rightarrow\sum\sqrt[3]{a^2}\ge\frac{4\sqrt[6]{2}\sum\sqrt{a}-3\sqrt[3]{4}}{3}\ge3\sqrt[3]{4}\)
\(\sum\left(\sqrt[3]{\frac{a^2}{16}}+\frac{1}{\sqrt[3]{a^2}}\right)\ge6\sqrt[6]{\frac{1}{16}}=\frac{6}{\sqrt[3]{4}}\)
\(\Rightarrow P\ge\frac{15}{\sqrt[3]{16}}.3\sqrt[3]{4}+\frac{6}{\sqrt[3]{4}}=\frac{51}{\sqrt[3]{4}}=3.\sqrt[3]{\frac{17^3}{4}}\)
\(\Rightarrow S\ge3\sqrt[3]{\frac{17^3}{4}}:\sqrt[3]{17^2}=3\sqrt[3]{\frac{17}{4}}\)
Dấu "=" xảy ra khi \(a=b=c=2\)
Bài toán nhạt nhẽo, chẳng có gì ngoài tính trâu, lần sau xin né :(
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\(S\ge3\frac{1}{\sqrt[6]{\left(a+2b+5c\right)\left(b+2c+5a\right)\left(c+2a+5b\right)}}.\)
\(S\ge\frac{3.4}{\sqrt[6]{\left(a+2b+5c\right)\left(b+2c+5a\right)\left(c+2a+5b\right).16.16.16}}\)
\(S\ge\frac{12}{\frac{a+2b+5c+b+2c+5a+c+2a+5b+16+16+16}{6}}\)
\(S\ge\frac{3}{4}\)
Dấu "=" xảy ra \(\Leftrightarrow a=b=c=2\)
![](https://rs.olm.vn/images/avt/0.png?1311)
1.
\(6=\frac{\sqrt{2}^2}{x}+\frac{\sqrt{3}^2}{y}\ge\frac{\left(\sqrt{2}+\sqrt{3}\right)^2}{x+y}=\frac{5+2\sqrt{6}}{x+y}\)
\(\Rightarrow x+y\ge\frac{5+2\sqrt{6}}{6}\)
Dấu "=" xảy ra khi \(\left\{{}\begin{matrix}\frac{x}{\sqrt{2}}=\frac{y}{\sqrt{3}}\\x+y=\frac{5+2\sqrt{6}}{6}\end{matrix}\right.\)
Bạn tự giải hệ tìm điểm rơi nếu thích, số xấu quá
2.
\(VT\ge\sqrt{\left(x+y+z\right)^2+\left(\frac{1}{x}+\frac{1}{y}+\frac{1}{z}\right)^2}\ge\sqrt{\left(x+y+z\right)^2+\frac{81}{\left(x+y+z\right)^2}}\)
Đặt \(x+y+z=t\Rightarrow0< t\le1\)
\(VT\ge\sqrt{t^2+\frac{81}{t^2}}=\sqrt{t^2+\frac{1}{t^2}+\frac{80}{t^2}}\ge\sqrt{2\sqrt{\frac{t^2}{t^2}}+\frac{80}{1^2}}=\sqrt{82}\)
Dấu "=" xảy ra khi \(x=y=z=\frac{1}{3}\)
3.
\(\frac{a^2}{b^5}+\frac{a^2}{b^5}+\frac{a^2}{b^5}+\frac{1}{a^3}+\frac{1}{a^3}\ge5\sqrt[5]{\frac{a^6}{b^{15}.a^6}}=\frac{5}{b^3}\)
Tương tự: \(\frac{3b^2}{c^5}+\frac{2}{b^3}\ge\frac{5}{a^3}\) ; \(\frac{3c^2}{d^5}+\frac{2}{c^3}\ge\frac{5}{d^3}\) ; \(\frac{3d^2}{a^5}+\frac{2}{d^2}\ge\frac{5}{a^3}\)
Cộng vế với vế và rút gọn ta được: \(3VT\ge3VP\)
Dấu "=" xảy ra khi và chỉ khi \(a=b=c=d=1\)
4.
ĐKXĐ: \(-2\le x\le2\)
\(y^2=\left(x+\sqrt{4-x^2}\right)^2\le2\left(x^2+4-x^2\right)=8\)
\(\Rightarrow y\le2\sqrt{2}\Rightarrow y_{max}=2\sqrt{2}\) khi \(x=\sqrt{2}\)
Mặt khác do \(\left\{{}\begin{matrix}x\ge-2\\\sqrt{4-x^2}\ge0\end{matrix}\right.\) \(\Rightarrow x+\sqrt{4-x^2}\ge-2\)
\(y_{min}=-2\) khi \(x=-2\)
Ta có: \(S^2=\frac{a^2}{b}+\frac{b^2}{c}+\frac{c^2}{a}+2\frac{a\sqrt{b}}{\sqrt{c}}+2\frac{b\sqrt{c}}{\sqrt{a}}+2\frac{c\sqrt{a}}{\sqrt{b}}\)
Áp dụng BĐT Cosi cho 3 số dương ta được
\(\hept{\begin{cases}\frac{a^2}{b}+\frac{a\sqrt{b}}{\sqrt{c}}+\frac{a\sqrt{b}}{\sqrt{c}}+c\ge4a\left(1\right)\\\frac{b^2}{c}+\frac{b\sqrt{c}}{\sqrt{a}}+\frac{b\sqrt{c}}{a}+a\ge4b\left(2\right)\\\frac{c^2}{a}+\frac{c\sqrt{a}}{\sqrt{b}}+\frac{c\sqrt{a}}{\sqrt{b}}+b\ge4c\left(3\right)\end{cases}}\)
Cộng theo từng vế của (1) (2) (3)
=> \(S^2\ge3\left(a+b+c\right)\ge9\Rightarrow A\ge3\)
=> MinS=3 đạt được khi a=b=c=1