Lời giải:
Đặt $x+7=t$ thì:

$P=(x+8)^4+(x+6)^4=(t+1)^4+(t-1)^4=2t^4+12t^2+2\geq 2, \forall t\in\mathbb{R}$

Do đó $P_{\min}=2$.

Giá trị này đạt tại $t=0\Leftrightarrow x+7=0$

$\Leftrightarrow x=-7$

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

\(A=x^2-4x+4-3\)

\(A=\left(x^2-4x+4\right)-3\)

\(A=\left(x-2\right)^2-3\)

Ta có: \(\left(x-2\right)^2\ge0\) với mọi x

\(\Rightarrow\left(x-2\right)^2-3\ge-3\) với mọi x

Vậy MIinA = -3 khi x = 2

2) \(B=-x^2+13x+2012\)

\(B=-x^2+13x-\frac{169}{4}+\frac{169}{4}+2012\)

\(B=-\left(x^2-13+\frac{169}{4}\right)+\left(\frac{169}{4}+2012\right)\)

\(B=-\left(x-\frac{13}{2}\right)^2+\frac{8217}{4}\)

Ta có: \(\left(x-\frac{13}{2}\right)^2\ge0\) với mọi x

\(-\left(x-\frac{13}{2}\right)^2\le0\) với mọi x

\(\Rightarrow-\left(x-\frac{13}{2}\right)^2+\frac{8217}{4}\le\frac{8217}{4}\)

Vây \(Max\left(B\right)=\frac{8217}{4}\) khi \(x=\frac{13}{2}\)

Lời giải:
Ta có:

$P=2x^2+y^2+2xy+5x+y+\frac{37}{4}$

$=(x^2+y^2+2xy)+x^2+5x+y+\frac{37}{4}$

$=(x+y)^2+(x+y)+(x^2+4x)+\frac{37}{4}$

$=(x+y)^2+(x+y)+\frac{1}{4}+(x^2+4x+4)+5$

$=(x+y+\frac{1}{2})^2+(x+2)^2+5\geq 5$

Vậy $P_{\min}=5$. Giá trị này đạt tại:

$x+y+\frac{1}{2}=x+2=0$

$\Leftrightarrow x=-2; y=\frac{3}{2}$

Lời giải:
Ta có:

$P=2x^2+y^2+2xy+5x+y+\frac{37}{4}$

$=(x^2+y^2+2xy)+x^2+5x+y+\frac{37}{4}$

$=(x+y)^2+(x+y)+(x^2+4x)+\frac{37}{4}$

$=(x+y)^2+(x+y)+\frac{1}{4}+(x^2+4x+4)+5$

$=(x+y+\frac{1}{2})^2+(x+2)^2+5\geq 5$

Vậy $P_{\min}=5$. Giá trị này đạt tại:

$x+y+\frac{1}{2}=x+2=0$

$\Leftrightarrow x=-2; y=\frac{3}{2}$

a. giá trị nhỏ nhất của B=3 khi và chỉ khi x=y=1006

Bài 1:

a: \(M=x^2-10x+3\)

\(=x^2-10x+25-22\)

\(=\left(x^2-10x+25\right)-22\)

\(=\left(x-5\right)^2-22>=-22\forall x\)

Dấu '=' xảy ra khi x-5=0

=>x=5

b: \(N=x^2-x+2\)

\(=x^2-x+\dfrac{1}{4}+\dfrac{7}{4}\)

\(=\left(x-\dfrac{1}{2}\right)^2+\dfrac{7}{4}>=\dfrac{7}{4}\forall x\)

Dấu '=' xảy ra khi x-1/2=0

=>x=1/2

c: \(P=3x^2-12x\)

\(=3\left(x^2-4x\right)\)

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

\(=3\left(x-2\right)^2-12>=-12\forall x\)

Dấu '=' xảy ra khi x-2=0

=>x=2

ba cái đề bài mn help e vs

\(K=-x^2+13x+2012=x^2+13x-\frac{169}{4}+\frac{8217}{4}\)

\(=\left(-x^2+13x-\frac{169}{4}\right)+\frac{8217}{4}\)

Mà \(-x^2+13x-\frac{169}{4}=2x\left(-\frac{1}{2}x+\frac{13}{2}\right)-\frac{169}{4}\le0\) ( do \(2x\left(-\frac{1}{2}x+\frac{13}{2}\right)\le\frac{169}{4}\))

Do đó \(K=\left(-x^2+13x-\frac{169}{4}\right)+\frac{8217}{4}\le\frac{8217}{4}\)

Dấu "=" xảy ra \(\Leftrightarrow2x\left(-\frac{1}{2}x+\frac{13}{2}\right)=\frac{169}{4}\Leftrightarrow x=\frac{13}{2}\)

Vậy \(K_{max}=\frac{8217}{4}\Leftrightarrow x=\frac{13}{2}\)

Tìm giá trị nhỏ nhất của biểu thức:

a) Ta có: 

\(M=2x^2+4x+7\)

\(M=2\cdot\left(x^2+2x+\dfrac{7}{2}\right)\)

\(M=2\cdot\left(x^2+2x+1+\dfrac{5}{2}\right)\)

\(M=2\cdot\left[\left(x+1\right)^2+2,5\right]\)

\(M=2\left(x+1\right)^2+5\)

Mà: \(2\left(x+1\right)^2\ge0\forall x\) nên:

\(M=2\left(x+1\right)^2+5\ge5\forall x\)

Dấu "=" xảy ra:

\(2\left(x+1\right)^2+5=5\Leftrightarrow2\left(x+1\right)^2=0\)

\(\Leftrightarrow\left(x+1\right)^2=0\Leftrightarrow x+1=0\Leftrightarrow x=-1\)

Vậy: \(M_{min}=5\) khi \(x=-1\)

b) Ta có:

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

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

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

Mà: \(\left(x+\dfrac{1}{2}\right)^2\ge0\forall x\) nên \(N=\left(x-\dfrac{1}{2}\right)^2+\dfrac{3}{4}\ge\dfrac{3}{4}\forall x\)

Dấu '=" xảy ra: 

\(\left(x-\dfrac{1}{2}\right)^2+\dfrac{3}{4}=\dfrac{3}{4}\Leftrightarrow\left(x-\dfrac{1}{2}\right)^2=0\)

\(\Leftrightarrow x-\dfrac{1}{2}=0\Leftrightarrow x=\dfrac{1}{2}\)

Vậy: \(N_{min}=\dfrac{3}{4}\) khi \(x=\dfrac{1}{2}\)

Tìm giá trị lớn nhất của biểu thức

a) Ta có: 

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

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

\(E=-\left[\left(2x\right)^2-2\cdot2x\cdot\dfrac{1}{4}+\dfrac{1}{16}+\dfrac{15}{16}\right]\)

\(E=-\left[\left(2x-\dfrac{1}{4}\right)^2+\dfrac{15}{16}\right]\)

Mà: \(\left(2x+\dfrac{1}{4}\right)^2+\dfrac{15}{16}\ge\dfrac{15}{16}\forall x\) nên 

\(\Rightarrow E=-\left[\left(2x+\dfrac{1}{4}\right)^2+\dfrac{15}{16}\right]\le-\dfrac{15}{16}\forall x\)

Dấu "=" xảy ra:

\(-\left[\left(2x+\dfrac{1}{4}\right)^2+\dfrac{15}{16}\right]=-\dfrac{15}{16}\Leftrightarrow-\left(2x+\dfrac{1}{4}\right)^2-\dfrac{15}{16}=-\dfrac{15}{16}\)

\(\Leftrightarrow-\left(2x+\dfrac{1}{4}\right)^2=0\Leftrightarrow2x-\dfrac{1}{4}=0\Leftrightarrow x=\dfrac{1}{16}\)

Vậy: \(E_{max}=-\dfrac{15}{16}\) khi \(x=\dfrac{1}{16}\)

b) Ta có:

\(F=5x-3x^2+6\)

\(F=-3x^2+5x-6\)

\(F=-\left(3x^2-5x-6\right)\)

\(F=-3\left(x^2-\dfrac{5}{3}x-2\right)\)

\(F=-3\left[\left(x-\dfrac{5}{6}\right)^2-\dfrac{97}{36}\right]\)

\(F=-3\left(x-\dfrac{5}{6}\right)^2+\dfrac{97}{36}\)

Mà: \(-3\left(x-\dfrac{5}{6}\right)^2\le0\forall x\) nên:

\(F=-3\left(x-\dfrac{5}{6}\right)^2+\dfrac{97}{36}\le\dfrac{97}{36}\forall x\)

Dấu "=" xảy ra:

\(-3\left(x-\dfrac{5}{6}\right)^2+\dfrac{97}{36}=\dfrac{97}{36}\Leftrightarrow-3\left(x-\dfrac{5}{6}\right)^2=0\)

\(\Leftrightarrow x-\dfrac{5}{6}=0\Leftrightarrow x=\dfrac{5}{6}\)

Vậy: \(F_{max}=\dfrac{97}{36}\) khi \(x=\dfrac{5}{6}\)

`A=x^4-6x^3+18x^2-6xy+y^2+2012`
`=x^4-6x^3+9x^2+9x^2-6xy+y^2+2012`
`=(x^2-x)^2+(3x-y)^2+2012>=2012`
Dấu "=" xảy ra khi:
$\begin{cases}x=x^2\\y=3x\end{cases}$
`<=>` $\left[ \begin{array}{l}\begin{cases}x=0\\y=3x=0\\\end{cases}\\\begin{cases}x=1\\y=3x=3\\\end{cases}\end{array} \right.$
Vậy `min_A=2012<=>` $\left[ \begin{array}{l}x=y=0\\\begin{cases}x=1\\y=3\end{cases}\end{array} \right.$