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Áp dụng BĐT \(\sqrt{xy}\le\frac{x+y}{2}\)
\(VT=\frac{2\left(a+b+c\right)}{\sqrt{4a\left(a+3b\right)}+\sqrt{4b\left(b+3c\right)}+\sqrt{4c\left(c+3a\right)}}\)
\(\Rightarrow VT\ge\frac{2\left(a+b+c\right)}{\frac{4a+a+3b}{2}+\frac{4b+b+3c}{2}+\frac{4c+c+3a}{2}}\)
\(\Rightarrow VT\ge\frac{4\left(a+b+c\right)}{8\left(a+b+c\right)}=\frac{1}{2}\) (đpcm)
Dấu "=" khi \(a=b=c\)
Lời giải:
BĐT cần chứng minh tương đương với:
\(\frac{bc}{\sqrt{5abc(3a+2b)}}+\frac{ac}{\sqrt{5abc(3b+2c)}}+\frac{ab}{\sqrt{5abc(3c+2a)}}\geq \frac{3}{5}(*)\)
Áp dụng BĐT AM-GM:
\(5abc(3a+2b)=5ab.(3ac+2bc)\leq \left(\frac{5ab+3ac+2bc}{2}\right)^2\)
\(\Rightarrow \frac{bc}{\sqrt{5abc(3a+2b)}}\geq \frac{2bc}{5ab+3ac+2bc}=\frac{2(bc)^2}{5ab^2c+3abc^2+2b^2c^2}\)
Hoàn toàn tương tự với các phân thức còn lại, cộng theo vế ta suy ra:
\(\sum \frac{bc}{\sqrt{5abc(3a+2b)}}\geq \sum \frac{2(bc)^2}{5ab^2c+3abc^2+2b^2c^2}(1)\)
Áp dụng BĐT Cauchy_Schwarz và AM-GM:
\(\sum \frac{2(bc)^2}{5ab^2c+3abc^2+2b^2c^2}\geq 2.\frac{(bc+ab+ac)^2}{2[(ab)^2+(bc)^2+(ca)^2+4abc(a+b+c)]}=\frac{(ab+bc+ac)^2}{(ab)^2+(bc)^2+(ca)^2+4abc(a+b+c)}\)
\(=\frac{(ab+bc+ac)^2}{(ab+bc+ac)^2+2abc(a+b+c)}\geq \frac{(ab+bc+ac)^2}{(ab+bc+ac)^2+\frac{2}{3}(ab+bc+ac)^2}=\frac{3}{5}(2)\)
Từ $(1);(2)$ suy ra $(*)$ đúng. BĐT được chứng minh.
Dấu "=" xảy ra khi $a=b=c$
2. Bạn kiểm tra lại đề: VP = 1/2
Ta có:
\(\sqrt{a\left(3a+b\right)}=\frac{1}{4}.2.\sqrt{4a\left(3a+b\right)}\le\frac{1}{4}\left(4a+3a+b\right)=\frac{1}{4}\left(7a+b\right)\)
\(\sqrt{b\left(3b+a\right)}=\frac{1}{4}.2.\sqrt{4b\left(3b+a\right)}\le\frac{1}{4}\left(4b+3b+a\right)=\frac{1}{4}\left(7b+a\right)\)
=> \(\frac{a+b}{\sqrt{a\left(3a+b\right)}+\sqrt{b\left(3b+a\right)}}\ge\frac{a+b}{\frac{1}{4}\left(7a+b\right)+\frac{1}{4}\left(7b+a\right)}=\frac{a+b}{2\left(a+b\right)}=\frac{1}{2}\)
Vậy: \(\frac{a+b}{\sqrt{a\left(3a+b\right)}+\sqrt{b\left(3b+a\right)}}\ge\frac{1}{2}\) với a, b dương
\(\Leftrightarrow\frac{\sqrt{bc}}{\sqrt{5a\left(3a+2b\right)}}+\frac{\sqrt{ac}}{\sqrt{5b\left(3b+2c\right)}}+\frac{\sqrt{ab}}{\sqrt{5c\left(3c+2a\right)}}\ge\frac{3}{5}\)
\(\Leftrightarrow\frac{bc}{\sqrt{5ab\left(3ac+2bc\right)}}+\frac{ac}{\sqrt{5bc\left(3ab+2ac\right)}}+\frac{ab}{\sqrt{5ac\left(3bc+2ab\right)}}\ge\frac{3}{5}\)
Thật vậy, theo AM-GM ta có:
\(VT\ge\frac{2bc}{5ab+2bc+3ac}+\frac{2ac}{3ab+5bc+2ac}+\frac{2ab}{2ab+3bc+5ac}\)
Đặt \(\left(ab;bc;ca\right)=\left(x;y;z\right)\)
\(\Rightarrow VT\ge\frac{2x}{2x+3y+5z}+\frac{2y}{5x+2y+3z}+\frac{2z}{3x+5y+2z}=\frac{2x^2}{2x^2+3xy+5zx}+\frac{2y^2}{5xy+2y^2+3yz}+\frac{2z^2}{3zx+5yz+2z^2}\)
\(\Rightarrow VT\ge\frac{\left(x+y+z\right)^2}{\left(x^2+y^2+z^2+2xy+2yz+2zx\right)+2\left(xy+yz+zx\right)}=\frac{\left(x+y+z\right)^2}{\left(x+y+z\right)^2+2\left(xy+yz+zx\right)}\)
\(\Rightarrow VT\ge\frac{\left(x+y+z\right)^2}{\left(x+y+z\right)^2+\frac{2}{3}\left(x+y+z\right)^2}=\frac{3}{5}\) (đpcm)
Dấu "=" xảy ra khi \(x=y=z\) hay \(a=b=c\)
Sửa đề: \(\frac{a}{b}+\frac{a}{c}+\frac{c}{b}+\frac{c}{a}+\frac{b}{c}+\frac{b}{a}\ge\sqrt{2}\left(\Sigma\sqrt{\frac{1-a}{a}}\right)\)
or \(\Sigma\frac{b+c}{a}\ge\Sigma\sqrt{\frac{2\left(b+c\right)}{a}}\)
Theo AM-GM:\(\frac{b+c}{a}\ge2\sqrt{\frac{2\left(b+c\right)}{a}}-2\)
Tương tự và cộng lại: \(VT\ge2\Sigma\sqrt{\frac{2\left(b+c\right)}{a}}-6\)
Mà: \(\Sigma\sqrt{\frac{2\left(b+c\right)}{a}}\ge3\sqrt[6]{\frac{8\left(a+b\right)\left(b+c\right)\left(c+a\right)}{abc}}\ge6\)
Từ đó: \(VT\ge2\Sigma\sqrt{\frac{2\left(b+c\right)}{a}}-\Sigma\sqrt{\frac{2\left(b+c\right)}{a}}=VP\)
Done!
