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\(P=\frac{1}{25a}+\frac{1}{16b}+\frac{1}{9c}=\frac{\frac{1}{25}}{a}+\frac{\frac{1}{16}}{b}+\frac{\frac{1}{9}}{c}\ge\frac{\left(\frac{1}{5}+\frac{1}{4}+\frac{1}{3}\right)^2}{a+b+c}=\frac{2209}{3600}\)
Dấu "=" xảy ra \(\Leftrightarrow\)\(\frac{\frac{1}{5}}{a}=\frac{\frac{1}{4}}{b}=\frac{\frac{1}{3}}{c}=\frac{\frac{1}{5}+\frac{1}{4}+\frac{1}{3}}{a+b+c}=\frac{47}{60}\)
\(\Rightarrow\)\(\hept{\begin{cases}a=\frac{1}{5}:\frac{47}{60}=\frac{12}{47}\\b=\frac{1}{4}:\frac{47}{60}=\frac{15}{47}\\c=\frac{1}{3}:\frac{47}{60}=\frac{20}{47}\end{cases}}\)
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Đặt \(x=a+b+2c;y=2a+b+c;z=a+b+3c\left(x,y,z>0\right)\)
Từ đó tính được: \(\hept{\begin{cases}a=z+y-2x\\b=5x-y-3z\\c=z-x\end{cases}}\)
Lúc đó \(A=\frac{4\left(z+y-2x\right)}{x}+\frac{\left(5x-y-3z\right)+3\left(z-x\right)}{y}-\frac{8\left(z-x\right)}{z}\)
\(=\frac{4z+4y}{x}-8+\frac{2x}{y}-1+\frac{8x}{z}-8\)
\(=\left(\frac{4y}{x}+\frac{2x}{y}\right)+\left(\frac{4z}{x}+\frac{8x}{z}\right)-17\)
\(\ge2\sqrt{\frac{4y}{x}.\frac{2x}{y}}+2\sqrt{\frac{4z}{x}.\frac{8x}{z}}-17=12\sqrt{2}-17\)(Theo BĐT Cô - si cho 2 số dương)
Đẳng thức xảy ra khi \(\hept{\begin{cases}\frac{4y}{x}=\frac{2x}{y}\\\frac{4z}{x}=\frac{8x}{z}\end{cases}}\Leftrightarrow\hept{\begin{cases}x=y\sqrt{2}\\z=x\sqrt{2}=2y\end{cases}}\Leftrightarrow\frac{z}{2}=\frac{x}{\sqrt{2}}=\frac{y}{1}\)
Đặt \(\frac{z}{2}=\frac{x}{\sqrt{2}}=\frac{y}{1}=k\left(k>0\right)\)thì \(\hept{\begin{cases}z=2k\\x=\sqrt{2}k\\y=k\end{cases}}\). Lúc đó \(\hept{\begin{cases}a=\left(3-2\sqrt{2}\right)k\\b=\left(5\sqrt{2}-7\right)k\\c=\left(2-\sqrt{2}\right)k\end{cases}}\)
Vậy \(MinA=12\sqrt{2}-17\), đạt được khi \(\hept{\begin{cases}a=\left(3-2\sqrt{2}\right)k\\b=\left(5\sqrt{2}-7\right)k\\c=\left(2-\sqrt{2}\right)k\end{cases}}\left(k>0\right)\)
Ta có
\(4\left(a+b+c+d\right)^2=\left(\left(a+b\right)+\left(b+c\right)+\left(c+d\right)+\left(d+a\right)\right)^2\)
\(=\left(\frac{\sqrt{a+b}}{\sqrt{b+c+d}}.\sqrt{a+b}.\sqrt{b+c+d}+\frac{\sqrt{b+c}}{\sqrt{c+d+a}}.\sqrt{b+c}.\sqrt{c+d+a}+\frac{\sqrt{c+d}}{\sqrt{d+a+b}}.\sqrt{c+d}.\sqrt{d+a+b}+\frac{\sqrt{d+a}}{\sqrt{a+b+c}}.\sqrt{d+a}.\sqrt{a+b+c}\right)^2\)
\(\le\left(\frac{a+b}{b+c+d}+\frac{b+c}{c+d+a}+\frac{c+d}{d+a+b}+\frac{d+a}{a+b+c}\right)\left(\left(a+b\right)\left(b+c+d\right)+\left(b+c\right)\left(c+d+a\right)+\left(c+d\right)\left(d+a+b\right)+\left(d+a\right)\left(a+b+c\right)\right)\)
\(\Rightarrow VT\ge\frac{4\left(a+b+c+d\right)^2}{\left(\left(a+b\right)\left(b+c+d\right)+\left(b+c\right)\left(c+d+a\right)+\left(c+d\right)\left(d+a+b\right)+\left(d+a\right)\left(a+b+c\right)\right)}\)(1)
Ta chứng minh
\(4\left(a+b+c+d\right)^2\ge\frac{8}{3}\left(\left(a+b\right)\left(b+c+d\right)+\left(b+c\right)\left(c+d+a\right)+\left(c+d\right)\left(d+a+b\right)+\left(d+a\right)\left(a+b+c\right)\right)\left(2\right)\)
\(\Leftrightarrow a^2+b^2+c^2+d^2-2ac-2bd\ge0\)
\(\Leftrightarrow\left(a-c\right)^2+\left(b-d\right)^2\ge0\)(đúng)
Từ (1) và (2) ta
\(\Rightarrow\frac{a+b}{b+c+d}+\frac{b+c}{c+d+a}+\frac{c+d}{d+a+b}+\frac{d+a}{a+b+c}\ge\frac{8}{3}\)
Dấu = xảy ra khi a = b = c = d
Bít làm thì đã làm rùi
khó quá