a) Từ M = x − 3 2 2 + 31 4 ≥ 31 4 ⇒ M min = 31 4 ⇔ x = 3 2 .  

b) Ta có N = ( x   +   2 y ) 2   +   ( y   –   2 ) 2   +   ( x   +   4 ) 2   –   120   ≥   -   120 .

Tìm được N min  = -120 Û x = -4 và y = 2.

Bài 3: 

a) Ta có: \(A=25x^2-20x+7\)

\(=\left(5x\right)^2-2\cdot5x\cdot2+4+3\)

\(=\left(5x-2\right)^2+3>0\forall x\)(đpcm)

d) Ta có: \(D=x^2-2x+2\)

\(=x^2-2x+1+1\)

\(=\left(x-1\right)^2+1>0\forall x\)(đpcm)

Bài 1: 

a) Ta có: \(A=x^2-2x+5\)

\(=x^2-2x+1+4\)

\(=\left(x-1\right)^2+4\ge4\forall x\)

Dấu '=' xảy ra khi x=1

b) Ta có: \(B=x^2-x+1\)

\(=x^2-2\cdot x\cdot\dfrac{1}{2}+\dfrac{1}{4}+\dfrac{3}{4}\)

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

Dấu '=' xảy ra khi \(x=\dfrac{1}{2}\)

Bài 6:

a) Ta có: \(A=-x^2+6x-11\)

\(=-\left(x^2-6x+11\right)\)

\(=-\left(x-3\right)^2-2\le-2\forall x\)

Dấu '=' xảy ra khi x=3

b) Ta có: \(B=-x^2-8x+5\)

\(=-\left(x^2+8x-5\right)\)

\(=-\left(x^2+8x+16-21\right)\)

\(=-\left(x+4\right)^2+21\le21\forall x\)

Dấu '=' xảy ra khi x=-4

c) Ta có: \(C=-x^2+4x+1\)

\(=-\left(x^2-4x-1\right)\)

\(=-\left(x^2-4x+4-5\right)\)

\(=-\left(x-2\right)^2+5\le5\forall x\)

Dấu '=' xảy ra khi x=2

Bài 7:

a) Ta có: \(x^2-6x+11\)

\(=x^2-6x+9+2\)

\(=\left(x-3\right)^2+2\ge2\forall x\)

Dấu '=' xảy ra khi x=3

\(A=\left(x^2-6x+9\right)+2=\left(x-3\right)^2+2\ge2\\ A_{min}=2\Leftrightarrow x=3\\ B=2\left(x^2-10x+25\right)+51=2\left(x-5\right)^2+51\ge51\\ B_{min}=51\Leftrightarrow x=5\\ C=\left[\left(x^2-4xy+4y^2\right)+10\left(x-2y\right)+25\right]+\left(y^2-2y+1\right)+2\\ C=\left(x-2y+5\right)^2+\left(y-1\right)^2+2\ge2\\ C_{min}=2\Leftrightarrow\left\{{}\begin{matrix}x-2y+5=0\\y-1=0\end{matrix}\right.\Leftrightarrow\left\{{}\begin{matrix}x=2y-5=2-5=-3\\y=1\end{matrix}\right.\)

a) \(A=\left(x^2-6x+9\right)+2=\left(x-3\right)^2+2\ge2\)

\(minA=2\Leftrightarrow x=3\)

b) \(B=2\left(x^2-10x+25\right)+51=2\left(x-5\right)^2+51\ge51\)

\(minB=51\Leftrightarrow x=5\)

c) \(C=\left[x^2-2x\left(2y-5\right)+\left(2y-5\right)^2\right]+\left(y^2-2y+1\right)+2=\left(x-2y+5\right)^2+\left(y-1\right)^2+2\ge2\)

\(minC=2\Leftrightarrow\left\{{}\begin{matrix}x=-3\\y=1\end{matrix}\right.\)

\(a,f\left(x\right)⋮g\left(x\right)\\ \Leftrightarrow\dfrac{-x^4+2x^2-3x+5}{x-1}\in Z\\ \Leftrightarrow\dfrac{-x^4+x^3-x^3+x^2+x^2-x-2x+2+3}{x-1}\in Z\\ \Leftrightarrow\dfrac{-x^3\left(x-1\right)-x^2\left(x-1\right)+x\left(x-1\right)-2\left(x-1\right)+3}{x-1}\in Z\\ \Leftrightarrow-x^3-x^2+x-2+\dfrac{3}{x-1}\in Z\\ \Leftrightarrow3⋮x-1\\ \Leftrightarrow x-1\inƯ\left(3\right)=\left\{-3;-1;1;3\right\}\\ \Leftrightarrow x\in\left\{-2;0;2;4\right\}\\ Mà.x< 0\\ \Leftrightarrow x=-2\\ b,B=\left(x^2-2xy+y^2\right)+4\left(x-y\right)+4+4y^2-2024\\ B=\left(x-y\right)^2+4\left(x-y\right)+4+4y^2-2024\\ B=\left(x-y-2\right)^2+4y^2-2024\ge-2024\\ B_{min}=-2024\Leftrightarrow\left\{{}\begin{matrix}x=y+2\\y=0\end{matrix}\right.\Leftrightarrow\left\{{}\begin{matrix}x=2\\y=0\end{matrix}\right.\)

\(a,A=\left(x^2-4xy+4y^2\right)+10\left(x-2y\right)+25+\left(y^2-2y+1\right)+2\\ A=\left(x-2y\right)^2+10\left(x-2y\right)+5+\left(y-1\right)^2+2\\ A=\left(x-2y+5\right)^2+\left(y-1\right)^2+2\ge2\)

Dấu \("="\Leftrightarrow\left\{{}\begin{matrix}x=2y-5\\y=1\end{matrix}\right.\Leftrightarrow\left\{{}\begin{matrix}x=-3\\y=1\end{matrix}\right.\)

\(b,\Leftrightarrow3x^3+10x^2-5+n=\left(3x+1\right)\cdot a\left(x\right)\)

Thay \(x=-\dfrac{1}{3}\Leftrightarrow3\left(-\dfrac{1}{27}\right)+10\cdot\dfrac{1}{9}-5+n=0\)

\(\Leftrightarrow-\dfrac{1}{9}+\dfrac{10}{9}-5+n=0\\ \Leftrightarrow-4+n=0\Leftrightarrow n=4\)

\(c,\Leftrightarrow2n^2-4n+5n-10+3⋮n-2\\ \Leftrightarrow2n\left(n-2\right)+5\left(n-2\right)+3⋮n-2\\ \Leftrightarrow n-2\inƯ\left(3\right)=\left\{-3;-1;1;3\right\}\\ \Leftrightarrow n\in\left\{-1;1;3;5\right\}\)

A = x^2 + 5y^2 + 4xy - 2y - 3 

= x^2 + 4xy + 4y^2 + y^2 - 2y + 1 - 4

= ( x + 2y )^2 + ( y - 1 )^2 - 4 >= -4 

Dấu ''='' xảy ra khi y = 1 ; x = -2 

Vậy GTNN A là -4 khi x = -2 ; y = 1

Ta có: \(A=x^2+5y^2+4xy-2y-3\)

\(=\left(x^2+4xy+4y^2\right)+\left(y^2-2y+1\right)-4\)

\(=\left(x+2y\right)^2+\left(y-1\right)^2-4\ge-4\forall x,y\)

Dấu '=' xảy ra khi \(\left\{{}\begin{matrix}y=1\\x=2\end{matrix}\right.\)

a: Ta có: \(A=x^2-20x+101\)

\(=x^2-20x+100+1\)

\(=\left(x-10\right)^2+1\ge1\forall x\)

Dấu '=' xảy ra khi x=10

Lời giải:

$A=(x^2+4y^2+4xy)+y^2+6x+16y+32$

$=(x+2y)^2+6(x+2y)+(y^2+4y)+32$

$=(x+2y)^2+6(x+2y)+9+(y^2+4y+4)+19$

$=(x+2y+3)^2+(y+2)^2+19\geq 0+0+19=19$

Vậy $A_{\min}=19$. Giá trị này đạt tại $x+2y+3=y+2=0$

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