tìm GTLN: -x^2+2xy-4y^2+2x+10y-8
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BS
1
26 tháng 6 2019
\(A=-x^2+2xy-4y^2+2x+10y-8\)
\(=-x^2+2xy-y^2-3y^2+2x-2y+12y-12+4\)
\(=-\left(x^2-2xy+y^2\right)+\left(2x-2y\right)-1-\left(3y^2-12y+12\right)+5\)
\(=-\left(x-y\right)^2+2\left(x-y\right)-1-3\left(y-2\right)^2+5\)
\(=-\left[\left(x-y\right)^2-2\left(x-y\right)+1\right]\)\(-3\left(y-2\right)^2+5\)
\(=-\left(x-y-1\right)^2-3\left(y-2\right)^2+5\)
\(A_{max}=5\Leftrightarrow\hept{\begin{cases}\left(x-y-1\right)^2=0\\3\left(y-2\right)^2=0\end{cases}\Rightarrow\hept{\begin{cases}x-y-1=0\\y-2=0\end{cases}}}\)
\(\Rightarrow\hept{\begin{cases}x-y-1=0\\y=2\end{cases}}\)\(\Rightarrow x-2-1=0\Leftrightarrow x=3\)
\(KL:A_{max}=5\Leftrightarrow x=3;y=2\)
KB
28 tháng 8 2018
\(-x^2+2xy-4y^2+2x+10y-8\)
\(=-\left(x^2-2xy+y^2\right)+2\left(x-y\right)+12y-8-3y^2\)
\(=-\left(x-y\right)^2+2\left(x-y\right)-3\left(y^2-4y+4\right)+4\)
\(=-\left[\left(x-y\right)^2-2\left(x-y\right)+1\right]-3\left(y-2\right)^2+5\)
\(=-\left(x-y-1\right)^2-3\left(y-2\right)^2+5\)
\(=-\left[\left(x-y-1\right)^2+3\left(y-2\right)^2\right]+5\le5\forall x;y\)
Dấu " = " xảy ra
\(\Leftrightarrow\left\{{}\begin{matrix}\left(x-y-1\right)^2=0\\3\left(y-2\right)^2=0\end{matrix}\right.\Leftrightarrow\left\{{}\begin{matrix}x-y-1=0\\\left(y-2\right)^2=0\end{matrix}\right.\)
\(\Leftrightarrow\left\{{}\begin{matrix}x-y=1\\y-2=0\end{matrix}\right.\Leftrightarrow\left\{{}\begin{matrix}x=y+1\\y=2\end{matrix}\right.\)
\(\Leftrightarrow\left\{{}\begin{matrix}x=3\\y=2\end{matrix}\right.\)
Vậy GTLN của biểu thức trên là : \(5\Leftrightarrow\left\{{}\begin{matrix}x=3\\y=2\end{matrix}\right.\)
26 tháng 9 2017
\(C=-x^2+2xy-4y^2+2x+10y-3\)
\(=-\left(x^2+2xy-y^2\right)+2x-2y-1-3y^2+12y-12+10\)
\(=-\left(x-y\right)^2+2\left(x-y\right)-1-3\left(y^2-4y+4\right)+10\)
\(=-\left(x-y-1\right)^2-3\left(y-2\right)^2+10\le10\forall x;y\)
Dấu "=" xảy ra \(\Leftrightarrow\hept{\begin{cases}x-y-1=0\\y-2=0\end{cases}\Leftrightarrow\hept{\begin{cases}x=3\\y=2\end{cases}}}\)
Vậy \(C_{max}=10\) tại x = 3; y = 2
5 tháng 9 2018
\(-x^2+2xy-4y^2+2x+10y-8\)
\(=-\left(x^2+2xy+y^2\right)-3y^2+2x+10y-8\)
\(=-\left(x-y\right)^2+2x-2y+12y-3y^2-8\)
\(=-\left(x-y\right)^2+2\left(x-y\right)-1+12y-3y^2-7\)
\(=-\left[\left(x-y\right)^2-2\left(x-y\right)+1\right]+12y-3y^2-7\)
\(=-\left(x-y-1\right)^2-3\left(y^2-4y+4\right)+5\)
\(=-\left(x-y-1\right)^2-3\left(y-2\right)^2+5\le5\)
\("="\Leftrightarrow\left\{{}\begin{matrix}x=3\\y=2\end{matrix}\right.\)
\(A=-x^2+2xy-4y^2+2x+10y-8\)
\(=-\left(x^2-2xy+4y^2-2x-10y+8\right)\)
\(=-\left[\left(x-y-1\right)^2+3\left(y-2\right)^2-5\right]\)
\(=5-\left(x-y-1\right)^2-3\left(y-2\right)^2\le5\)
Dấu"=" xảy ra <=> \(\hept{\begin{cases}x-y-1=0\\y-2=0\end{cases}}\) <=> \(\hept{\begin{cases}x=3\\y=2\end{cases}}\)
Vậy MAX \(A=5\)khi \(x=3;\)\(y=2\)