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TV
1
NV
Nguyễn Việt Lâm
Giáo viên
23 tháng 12 2020
\(x^2+2xy+y^2+6\left(x+y\right)+8=-y^2\)
\(\Leftrightarrow\left(x+y\right)^2+6\left(x+y\right)+8\le0\)
\(\Leftrightarrow\left(x+y+2\right)\left(x+y+4\right)\le0\)
\(\Rightarrow-4\le x+y\le-2\)
\(\Rightarrow2016\le B\le2018\)
\(B_{min}=2016\) khi \(\left(x;y\right)=\left(-4;0\right)\)
\(B_{max}=2018\) khi \(\left(x;y\right)=\left(-2;0\right)\)
NT
1
CC
5 tháng 11 2019
\(2x^2+2y^2+z^2+25-6y-2xy-8x+2z\left(y-x\right)=0\)
\(\Leftrightarrow\left(x^2-2xy+y^2\right)-2z\left(x-y\right)+z+\left(x^2-8x+16\right)+\left(y^2-6y+9\right)=0\)
\(\Leftrightarrow\left(x-y\right)^2-2z\left(x-y\right)+z^2+\left(x-4\right)^2+\left(y-3\right)^2=0\)
\(\Leftrightarrow\left(x-y-z\right)^2+\left(x-4\right)^2+\left(y-3\right)^2=0\)
\(\Leftrightarrow\hept{\begin{cases}x-y-z=0\\x-4=0\\y-3=0\end{cases}}\Leftrightarrow\hept{\begin{cases}z=1\\x=4\\y=3\end{cases}}\)
Vậy \(x=4\), \(y=3\), \(z=1\)
T
1
NV
Nguyễn Việt Lâm
Giáo viên
16 tháng 4 2021
\(B=\left(x^2+y^2+4+2xy-4x-4y\right)+\left(x^2+z^2+1+2xz-2x-2z\right)+\left(y^2-4y+4\right)+4\)
\(B=\left(x+y-2\right)^2+\left(x+z-1\right)^2+\left(y-2\right)^2+4\ge4\)
Dấu "=" xảy ra khi: \(\left\{{}\begin{matrix}x+y-2=0\\x+z-1=0\\y-2=0\end{matrix}\right.\) \(\Leftrightarrow\left\{{}\begin{matrix}x=0\\y=2\\z=1\end{matrix}\right.\)
NP
1
NV
Nguyễn Việt Lâm
Giáo viên
9 tháng 4 2021
\(\Leftrightarrow\left(x^2-2xy+y^2\right)+\left(y^2-2yz+z^2\right)+\left(z^2-2z+1\right)< 1\)
\(\Leftrightarrow\left(x-y\right)^2+\left(y-z\right)^2+\left(z-1\right)^2< 1\)
Nếu tồn tại 1 trong 3 số \(x-y;y-z;z-1\) khác 0
Do x; y; z nguyên
\(\Rightarrow\left(x-y\right)^2+\left(y-z\right)^2+\left(z-x\right)^2\ge1\) (vô lý)
\(\Rightarrow x-y=y-z=z-1=0\)
\(\Leftrightarrow x=y=z=1\)
NA
3
ND
Nguyễn Đức Trí
VIP
3 tháng 8 2023
\(\left\{{}\begin{matrix}3x-6y+2z=-4\\3x-y-3z=1\end{matrix}\right.\)
\(\Rightarrow\left\{{}\begin{matrix}3x-6y+2z=-4\\3x-y-3z=1\end{matrix}\right.\) \(\Rightarrow\left\{{}\begin{matrix}3x-6y=-4-2z\\3x-y=1+3z\end{matrix}\right.\)
\(\Rightarrow\left\{{}\begin{matrix}5y=1+3z+4+2z\\3x-y=1+3z\end{matrix}\right.\) \(\Rightarrow\left\{{}\begin{matrix}5y=5+5z\\3x=y+1+3z\end{matrix}\right.\)
\(\Rightarrow\left\{{}\begin{matrix}y=1+z\\3x=1+z+1+3z\end{matrix}\right.\) \(\Rightarrow\left\{{}\begin{matrix}y=1+z\\x=\dfrac{4z+6}{3}\end{matrix}\right.\)
\(S=9x^2-8\left(y^2+z^2\right)\)
\(S=9\left(\dfrac{4z+2}{3}\right)^2-8\left[\left(1+z\right)^2+z^2\right]\)
\(S=9.\dfrac{16z^2+16z+4}{9}-8\left[1+2z+z^2+z^2\right]\)
\(S=16z^2+16z+4-8-16z-16z^2\)
\(S=-4\)
ND
Nguyễn Đức Trí
VIP
3 tháng 8 2023
Đính chính \(x=\dfrac{4z+2}{3}\) không phải \(x=\dfrac{4z+6}{3}\)
5 tháng 10 2021
\(A=\left(2x-1\right)^2+9\ge9\\ A_{min}=9\Leftrightarrow x=\dfrac{1}{2}\\ B=2\left(x^2-2\cdot\dfrac{3}{4}x+\dfrac{9}{16}\right)+\dfrac{1}{8}=2\left(x-\dfrac{3}{4}\right)^2+\dfrac{1}{8}\ge\dfrac{1}{8}\\ B_{min}=\dfrac{1}{8}\Leftrightarrow x=\dfrac{3}{4}\\ C=\left(4x^2+4xy+y^2\right)+2\left(2x+y\right)+1+\left(y^2+4y+4\right)-4\\ C=\left[\left(2x+y\right)^2+2\left(2x+y\right)+1\right]+\left(y+2\right)^2-4\\ C=\left(2x+y+1\right)^2+\left(y+2\right)^2-4\ge-4\\ C_{min}=-4\Leftrightarrow\left\{{}\begin{matrix}2x=-1-y\\y=-2\end{matrix}\right.\Leftrightarrow\left\{{}\begin{matrix}x=-\dfrac{3}{2}\\y=-2\end{matrix}\right.\)
\(D=\left(3x-1-2x\right)^2=\left(x-1\right)^2\ge0\\ D_{min}=0\Leftrightarrow x=1\\ G=\left(9x^2+6xy+y^2\right)+\left(y^2+4y+4\right)+1\\ G=\left(3x+y\right)^2+\left(y+2\right)^2+1\ge1\\ G_{min}=1\Leftrightarrow\left\{{}\begin{matrix}3x=-y\\y=-2\end{matrix}\right.\Leftrightarrow\left\{{}\begin{matrix}x=\dfrac{2}{3}\\y=-2\end{matrix}\right.\)
5 tháng 10 2021
\(H=\left(x^2-2xy+y^2\right)+\left(x^2+2x+1\right)+\left(2y^2+4y+2\right)+2\\ H=\left(x-y\right)^2+\left(x+1\right)^2+2\left(y+1\right)^2+2\ge2\\ H_{min}=2\Leftrightarrow\left\{{}\begin{matrix}x=y\\x=-1\\y=-1\end{matrix}\right.\Leftrightarrow x=y=-1\)
Ta luôn có \(\left(x-y\right)^2+\left(y-z\right)^2+\left(z-x\right)^2\ge0\)
\(\Leftrightarrow2x^2+2y^2+2z^2-2xy-2yz-2xz\ge0\\ \Leftrightarrow x^2+y^2+z^2\ge xy+yz+xz\\ \Leftrightarrow x^2+y^2+z^2+2xy+2yz+2xz\ge3xy+3yz+3xz\\ \Leftrightarrow\left(x+y+z\right)^2\ge3\left(xy+yz+xz\right)\\ \Leftrightarrow\dfrac{3^2}{3}\ge xy+yz+xz\\ \Leftrightarrow K\le3\\ K_{max}=3\Leftrightarrow x=y=z=1\)
Ta có: \(2x^2+2y^2+z^2+25-6y-2xy-8x+2z\left(y-x\right)=0\)
\(\Leftrightarrow\left(x^2-8x+16\right)+\left(y^2-6y+9\right)+\left(x^2-2xy+y^2\right)-2\left(x-y\right)z+z^2=0\)
\(\Leftrightarrow\left(x-4\right)^2+\left(y-3\right)^2+\left[\left(x-y\right)^2-2\left(x-y\right)z+z^2\right]=0\)
\(\Leftrightarrow\left(x-4\right)^2+\left(y-3\right)^2+\left(x-y-z\right)^2=0\)
\(\Leftrightarrow\hept{\begin{cases}\left(x-4\right)^2=0\\\left(y-3\right)^2=0\\\left(x-y-z\right)^2=0\end{cases}}\Leftrightarrow\hept{\begin{cases}x=4\\y=3\\z=1\end{cases}}\)
Chỗ (x²-8x+16)
16 là ở đâu ra vậy bạn
Chỗ (y²-6y+9 )
9 là ở đâu ra nx v