2) \(A=-x^2-y^2+2x-6y+9=-\left(x^2-2x+1\right)-\left(y^2+6y+9\right)+19=-\left(x-1\right)^2-\left(y+3\right)^2+19\)

\(maxA=19\Leftrightarrow\)\(\left\{{}\begin{matrix}x=1\\y=-3\end{matrix}\right.\)

a: \(=\sqrt{x-3-2\sqrt{x-3}+3}\)

\(=\sqrt{x-3-2\sqrt{x-3}+1+2}=\sqrt{\left(\sqrt{x-3}-1\right)^2+2}>=\sqrt{2}\)

Dấu = xảy ra khi x-3=1

=>x=4

\(A=\left|3-x\right|+8\ge8\)

\(minA=8\Leftrightarrow x=3\)

\(B=\left|x+2\right|-4\ge-4\)

\(minB=-4\Leftrightarrow x=-2\)

\(A=\left|3-x\right|+8\ge8\forall x\)

Dấu '=' xảy ra khi x=3

\(B=\left|x+2\right|-4\ge-4\forall x\)

Dấu '=' xảy ra khi x=-2

$A=x^2+y^2-6x+4y+20=(x^2-6x+9)+(y^2+4y+4)+7$

$=(x-3)^2+(y+2)^2+7\geq 0+0+7=7$
Vậy $A_{\min}=7$. Giá trị này đạt tại $(x-3)^2=(y+2)^2=0$

$\Leftrightarrow x=3; y=-2$

---------------------

$B=9x^2+y^2+2z^2-18x+4z-6y+30$

$=(9x^2-18x+9)+(y^2-6y+9)+(2z^2+4z+2)+10$

$=9(x^2-2x+1)+(y^2-6y+9)+2(z^2+2z+1)+10$

$=9(x-1)^2+(y-3)^2+2(z+1)^2+10\geq 10$
Vậy $B_{\min}=10$. Giá trị này đạt tại $(x-1)^2=(y-3)^2=(z+1)^2$

$\Leftrightarrow x=1; y=3; z=-1$

$C=x^2+y^2+z^2-xy-yz-xz+3$

$2C=2x^2+2y^2+2z^2-2xy-2yz-2xz+6$

$=(x^2-2xy+y^2)+(y^2-2yz+z^2)+(x^2-2xz+z^2)+6$

$=(x-y)^2+(y-z)^2+(z-x)^2+6\geq 6$

$\Rightarrow C\geq 3$

Vậy $C_{\min}=3$. Giá trị này đạt tại $x-y=y-z=z-x=0$

$\Leftrihgtarrow x=y=z$

--------------------------------------

$D=5x^2+2y^2+4xy-2x+4y+2021$

$=2(y^2+2xy+x^2)+3x^2-2x+4y+2021$

$=2(x+y)^2+4(x+y)+3x^2-6x+2021$
$=2(x+y)^2+4(x+y)+2+3(x^2-2x+1)+2016$

$=2[(x+y)^2+2(x+y)+1]+3(x^2-2x+1)+2016$

$=2(x+y+1)^2+3(x-1)^2+2016\geq 2016$

Vậy $D_{\min}=2016$ khi $x+y+1=x-1=0$

$\Leftrightarrow x=1; y=-2$

$E=x^2-2x+4y^2+4y+2014$

$=(x^2-2x+1)+(4y^2+4y+1)+2012$

$=(x-1)^2+(2y+1)^2+2012$

$\geq 2012$

Vậy $E_{\min}=2012$. Giá trị này đạt tại $x-1=2y+1=0$

$\Leftrightarrow x=1; y=\frac{-1}{2}$

----------------------

$F=5x^2+5y^2+8xy+2y-2x+30$

$=4(x^2+2xy+y^2)+x^2+y^2+2y-2x+30$

$=4(x+y)^2+(x^2-2x+1)+(y^2+2y+1)+28$

$=4(x+y)^2+(x-1)^2+(y+1)^2+28\geq 28$

Vậy $F_{\min}=28$. Giá trị này đạt tại $x+y=x-1=y+1=0$

$\Leftrightarrow x=1; y=-1$

\(A=\left|x-201\right|+\left|x-204\right|=\left|x-201\right|+\left|204-x\right|\ge\left|x-201+204-x\right|=\left|3\right|=3\)

\(minA=3\Leftrightarrow\left(x-201\right)\left(204-x\right)\ge0\Leftrightarrow204\ge x\ge201\)

\(A=2a^2+b^2-2ab+10a+42=\left(a^2-2ab+b^2\right)+\left(a^2+10a+25\right)+17=\left(a-b\right)^2+\left(a+5\right)^2+17\ge17\)

\(minA=17\Leftrightarrow a=b=-5\)

\(A=\dfrac{x^2-4x+1}{x^2}=\dfrac{1}{x^2}-\dfrac{4}{x}+1=\left(\dfrac{1}{x^2}-\dfrac{4}{x}+4\right)-3=\left(\dfrac{1}{x}-2\right)^2-3\ge-3\)

\(A_{min}=-3\) khi \(x=\dfrac{1}{2}\)

Bài 1 : 

a) \(xy-2x+2y=10\)

\(\Leftrightarrow x\left(y-2\right)+2y=10\)

\(\Leftrightarrow x\left(y-2\right)+2y-4=6\)

\(\Leftrightarrow x\left(y-2\right)+2\left(y-2\right)=6\)

\(\Leftrightarrow\left(x+2\right)\left(y-2\right)=6\)

Ta có : \(x+2\ge2\) vì \(x\in N\)

Do đó : ta có bảng :

x+2 :  2       3      6

y-2 :    3       2       1

x    :     0        1       4

y    :      5        4        3

Vậy...........

a) \(xy-2x+2y=10\left(x;y\inℕ\right)\)

\(\Rightarrow2xy-4x+4y=20\)

\(\Rightarrow2x\left(y-2\right)+4y-8+8=20\)

\(\Rightarrow2x\left(y-2\right)+4\left(y-2\right)=12\)

\(\Rightarrow\left(2x+4\right)\left(y-2\right)=12\)

\(\Rightarrow\left(2x+4\right);\left(y-2\right)\in\left\{1;2;3;4;6;12\right\}\)

\(\Rightarrow\left(x;y\right)\in\left\{\left(-\dfrac{3}{2};14\right);\left(-1;8\right);\left(-\dfrac{1}{3};6\right);\left(0;5\right);\left(1;3\right);\left(4;3\right)\right\}\)

\(\Rightarrow\left(x;y\right)\in\left\{\left(0;5\right);\left(1;3\right);\left(4;3\right)\right\}\left(x;y\inℕ\right)\)

a, 7\(x\).(2\(x\) + 10) =0

    \(\left[{}\begin{matrix}x=0\\2x+10=0\end{matrix}\right.\)

    \(\left[{}\begin{matrix}x=0\\2x=-10\end{matrix}\right.\)

     \(\left[{}\begin{matrix}x=0\\x=-5\end{matrix}\right.\)

Vậy \(x\in\) {-5; 0}

b, -9\(x\) : (2\(x\) - 10) = 0

    9\(x\)                   = 0 

     \(x\)                    = 0 

c, (4 - \(x\)).(\(x\) + 3)  = 0

    \(\left[{}\begin{matrix}4-x=0\\x+3=0\end{matrix}\right.\)

    \(\left[{}\begin{matrix}x=4\\x=-3\end{matrix}\right.\)

Vậy \(x\in\) {-3; 4}

d, (\(x\) + 2023).(\(x\) - 2024) = 0

     \(\left[{}\begin{matrix}x+2023=0\\x-2024=0\end{matrix}\right.\)

     \(\left[{}\begin{matrix}x=-2023\\x=2024\end{matrix}\right.\)

Vậy \(x\in\) {-2023; 2024}