A,   -2x^2<,=0

4-2x^2<,=4

dấu = xảy ra <=> 2x^2=0

                     <=>x=0

vậy GTLN của A=4 đạt đc khi x=0

\(A=4-2x^2\le4\)(Vì \(x^2\ge0\))

Dấu '' = '' xảy ra khi: \(x=0\)

Vậy \(MaxA=4\Leftrightarrow x=0\)

\(B=-3x^2+2x-5\)

\(B=-3\left(x^2-\frac{2}{3}x+\frac{1}{9}\right)-\frac{14}{3}\)

\(B=-\left(x-\frac{1}{3}\right)^2-\frac{14}{3}\le\frac{-14}{3}\)

Dấu '' = '' xảy ra khi: 

\(x-\frac{1}{3}=0\)

\(\Leftrightarrow x=\frac{1}{3}\)

Vậy \(MaxB=\frac{-14}{3}\Leftrightarrow\frac{1}{3}\)

Câu 1 :

\(B=\left|3x-5\right|+\left|2-3x\right|\ge\left|3x-5+2-3x\right|=\left|-3\right|=3\)

Dấu "=" xảy ra  

TH1: \(\Leftrightarrow\hept{\begin{cases}3x-5>0\\2-3x>0\end{cases}\Leftrightarrow\hept{\begin{cases}x>\frac{5}{3}\\x< \frac{2}{3}\end{cases}\Rightarrow}\frac{5}{3}< x< \frac{2}{3}\left(\text{loại}\right)}\)

TH2: \(\Leftrightarrow\hept{\begin{cases}3x-5< 0\\2-3x< 0\end{cases}\Leftrightarrow\hept{\begin{cases}x< \frac{5}{3}\\x>\frac{2}{3}\end{cases}\Rightarrow}\frac{2}{3}< x< \frac{5}{3}\left(\text{thỏa mãn}\right)}\)

Vậy Bmin = 3 <=> 2/3 < x < 5/3 

Câu 2 :

\(C=\left|2x-20\right|-\left|2x+3\right|\le\left|2x-20-2x-3\right|=\left|-23\right|=23\)

Dấu "=" xảy ra 

TH1 : \(\Leftrightarrow\hept{\begin{cases}2x-20>0\\2x+3>0\end{cases}}\Leftrightarrow\hept{\begin{cases}x>10\\x>\frac{-3}{2}\end{cases}}\Rightarrow x>10\)

TH2: \(\Leftrightarrow\hept{\begin{cases}2x-20< 0\\2x+3< 0\end{cases}\Leftrightarrow\hept{\begin{cases}x< 10\\x< \frac{-3}{2}\end{cases}\Rightarrow}}x< \frac{-3}{2}\)

Vậy Cmax = 23 <=> 2 t/h ( ko chắc )

\(B=\left|3x-5\right|+\left|2-3x\right|\ge\left|3x-5+2-3x\right|=\left|-5+2\right|=3\)

Dấu "=" xảy ra \(\Leftrightarrow\left(3x-5\right)\left(2-3x\right)\ge0\)

                         \(\Leftrightarrow\hept{\begin{cases}3x-5\ge0\\2-3x\le0\end{cases}}\) hoặc   \(\hept{\begin{cases}3x-5\le0\\2-3x\ge0\end{cases}}\)

 Giải ra ta được: \(\Leftrightarrow\frac{2}{3}\le x\le\frac{5}{3}\)

Vậy B_min = 3 khi và chỉ khi \(\frac{2}{3}\le x\le\frac{5}{3}\)

_{\(C=\left|2x-20\right|-\left|2x+3\right|\le\left|2x-20-2x-3\right|=\left|-20-3\right|=23\)}

Dấu "=" xảy ra <=> \(\orbr{\begin{cases}2x-20\ge2x+3\ge0\\2x-20\le2x+3\le0\end{cases}}\Leftrightarrow\orbr{\begin{cases}x\ge10;x\ge\frac{-3}{2}\\x\le10;x\le\frac{-3}{2}\end{cases}}\)

Vậy C_max = 17 khi và chỉ khi ....

tao chịu

mk cx chịu lun

Câu 1:

\(M=x^2-3x+5\)

\(M=x^2-2.\frac{3}{2}x+\frac{9}{4}+\frac{11}{4}\)

\(M=\left(x-\frac{3}{2}\right)^2+\frac{11}{4}\ge\frac{11}{4}\)

            Dấu = xảy ra khi \(x-\frac{3}{2}=0\Rightarrow x=\frac{3}{2}\)

    Vậy Min M = 11/4 khi x=3/2

b)\(N=2x^2+3x\)

\(N=2\left(x^2+\frac{3}{2}x\right)\)

\(N=2\left(x^2+2.\frac{3}{4}x+\frac{9}{16}\right)-\frac{9}{8}\)

\(N=2\left(x+\frac{3}{4}\right)^2-\frac{9}{8}\ge-\frac{9}{8}\)

              Dấu = xảy ra khi \(x+\frac{3}{4}=0\Rightarrow x=-\frac{3}{4}\)

                       Vậy MIn N = -9/8 khi x=-3/4

c)Tự làm nha

Ta có : x² - 3x + 5 

= x² - 2.x.\(\frac{3}{2}\) + \(\frac{3}{2}^2\) + \(\frac{11}{4}\)

= \(\left(x-\frac{3}{2}\right)^2+\frac{11}{4}\)

Vì \(\left(x-\frac{3}{2}\right)^2\ge0\forall x\in R\)

Nên : \(\left(x-\frac{3}{2}\right)^2+\frac{11}{4}\) \(\ge\frac{11}{4}\forall x\in R\)

Vậy GTNN của biểu thức là : \(\frac{11}{4}\) khi \(x=\frac{3}{2}\)

Câu 2:

a)\(A=-x^2-5x+3\)

\(A=-\left(x^2+2.\frac{5}{2}x+\frac{25}{4}\right)+\frac{37}{4}\)

\(A=\frac{37}{4}-\left(x+\frac{5}{2}\right)^2\le\frac{37}{4}\)

            Dấu = xảy ra khi \(x+\frac{5}{2}=0\Rightarrow x=-\frac{5}{2}\)

                      Vậy Max A = 37/4 khi x=-5/2

b)\(B=-2x^2+3x\)

\(B=-2\left(x^2-\frac{3}{2}x\right)\)

\(B=-2\left(x^2-2.\frac{3}{4}+\frac{9}{16}\right)+\frac{9}{8}\)

\(B=\frac{9}{8}-2\left(x-\frac{3}{4}\right)^2\le\frac{9}{8}\)

         Dấu = xảy ra khi \(x-\frac{3}{4}=0\Rightarrow x=\frac{3}{4}\)

                    Vậy Max B=9/8 khi x=3/4

a) Ta có: \(\left|3x-5\right|\ge0\forall x\)

\(\Leftrightarrow-\left|3x-5\right|\le0\forall x\)

Dấu '=' xảy ra khi \(x=\dfrac{5}{3}\)

b) Ta có: \(\left|2x-3\right|\ge0\forall x\)

\(\Leftrightarrow-\left|2x-3\right|\le0\forall x\)

\(\Leftrightarrow-\left|2x-3\right|+1\le1\forall x\)

Dấu '=' xảy ra khi \(x=\dfrac{3}{2}\)

\(A=\left(2x+1\right)^2-\left(3x+2\right)^2+2x+11\)

\(=4x^2+4x+1-\left(9x^2+12x+4\right)+2x+11\)

\(=-5x^2-6x+8\)

\(=-5\left(x+\dfrac{3}{5}\right)^2+\dfrac{49}{5}\le\dfrac{49}{5}\)

\(A_{max}=\dfrac{49}{5}\) khi \(x=-\dfrac{3}{5}\)

A) \(A=-3x^2+x+1\)

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

\(A=-3\left(x^2-2\cdot\dfrac{1}{6}\cdot x+\dfrac{1}{36}-\dfrac{13}{36}\right)\)

\(A=-3\left(x-\dfrac{1}{6}\right)^2+\dfrac{13}{12}\)

Mà: \(-3\left(x-\dfrac{1}{6}\right)^2\le0\forall x\)

\(\Rightarrow A=-3\left(x-\dfrac{1}{6}\right)^2+\dfrac{13}{12}\le\dfrac{13}{12}\forall x\)

Dấu "=" xảy ra khi:

\(x-\dfrac{1}{6}=0\Rightarrow x=\dfrac{1}{6}\)

Vậy: \(A_{max}=\dfrac{13}{12}.khi.x=\dfrac{1}{6}\)

B) \(B=2x^2-8x+1\)

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

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

\(B=2\left(x-2\right)^2-7\)

Mà: \(2\left(x-2\right)^2\ge0\forall x\)

\(\Rightarrow B=2\left(x-2\right)^2-7\ge-7\forall x\)

Dấu "=" xảy ra khi:

\(x-2=0\Rightarrow x=2\)

Vậy: \(B_{min}=2.khi.x=2\)

câu a) bạn viết sai đề rồi

a,  Ta có: -4x²+4x-1=-(4x²-4x+1)<=>-((2x)²-2.2x+1)=-(2x-1)²

A = -4x² + 4x - 1

= -( 4x² - 4x + 1 )

= -( 2x - 1 )² ≤ 0 ∀ x

Đẳng thức xảy ra <=> 2x - 1 = 0 => x = 1/2

=> MaxA = 0 <=> x = 1/2

B = 3x² + 2x + 5

= 3( x² + 2/3x + 1/9 ) + 14/3

= 3( x + 1/3 )² + 14/3 ≥ 14/3 ∀ x

Đẳng thức xảy ra <=> x + 1/3 = 0 => x = -1/3

=> MinB = 14/3 <=> x = -1/3

Giải: Ta có:

B = \(\frac{3x^2-6x+17}{x^2-2x+5}=\frac{3\left(x^2-2x+1\right)+14}{\left(x^2-2x+1\right)+4}=\frac{3\left(x-1\right)^2+14}{\left(x-1\right)^2+4}=3+\frac{14}{\left(x-1\right)^2+4}\)

Do \(\left(x-1\right)^2\ge0\forall x\) => \(\left(x-1\right)^2+4\ge4\forall x\)

 => \(\frac{14}{\left(x-1\right)^2+4}\le\frac{7}{2}\forall x\) 

=> \(3+\frac{14}{\left(x-1\right)^2+4}\le\frac{13}{2}\forall x\)

Dấu "=" xảy ra <=> x - 1 = 0 <=> x = 1

Vậy MaxA = 13/2 <=> x = 1