SM

\(Chox,y>0\)

\(\log_{\sqrt{3}}\left[\dfrac{2x+y}{4x^2+y^2+2xy+2}\right]=2x\left(2x-3\right)+y\left(y-3\right)+2xy\)

Tính \(P_{Max}=\dfrac{6x+2y+1}{2x+y+6}\)


 

 

NL
6 tháng 7 2021 lúc 21:24

\(log_{\sqrt{3}}\left(2x+y\right)-log_{\sqrt{3}}\left(4x^2+y^2+2xy+2\right)=\left(4x^2+y^2+2xy+2\right)-3\left(2x+y\right)-2\)

\(\Leftrightarrow log_{\sqrt{3}}\left(2x+y\right)+2+3\left(2x+y\right)=log_{\sqrt{3}}\left(4x^2+y^2+2xy+2\right)+\left(4x^2+y^2+2xy+2\right)\)

\(\Leftrightarrow log_{\sqrt{3}}\left(6x+3y\right)+\left(6x+3y\right)=log_{\sqrt{3}}\left(4x^2+y^2+2xy+2\right)+\left(4x^2+y^2+2xy+2\right)\)

Xét hàm \(f\left(t\right)=log_{\sqrt{3}}t+t\) với \(t>0\)

\(f'\left(t\right)=\dfrac{1}{t.ln\sqrt{3}}+1>0\Rightarrow f\left(t\right)\) đồng biến

\(\Rightarrow6x+3y=4x^2+y^2+2xy+2\)

\(\Leftrightarrow4x+y=\left(x+y-1\right)^2+1+3\left(x^2+1\right)-3\ge2\left(x+y-1\right)+6x-3\)

\(\Leftrightarrow4x+y\ge2\left(4x+y\right)-5\)

\(\Leftrightarrow4x+y\le5\)

\(\Rightarrow P=\dfrac{2x+y+6+\left(4x+y-5\right)}{2x+y+6}=1+\dfrac{4x+y-5}{2x+y+6}\le1\)

\(P_{max}=1\) khi \(x=y=1\)

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