A=2x² + 3x + 1 

A=2(x² + 3/2x + 9/16) -1/8

A=2(x+3/4)² - 1/8

Vậy GTNN của A là -1/8

- 1 hay + 1 v?

+ 1 hợp lí hơn

\(A=3x^2-2x-1\)

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

\(A=3\left(x^2-2.\frac{1}{3}.x+\frac{1}{9}-\frac{4}{9}\right)\)

\(A=3\left(x-\frac{1}{3}\right)^2-\frac{4}{3}\ge-\frac{4}{3}\)với \(\forall x\)

Dấu "=" xảy ra <=> x=1/3

Vậy GTNN của A là -4/3 <=> x=1/3

a) \(A=2x^2\)\(+\)\(10\)\(-\)\(1\)

\(=2\left(x^2+5x-\frac{1}{2}\right)\)

\(=2\left(x^2+2.x.\frac{5}{2}+\frac{25}{4}-\frac{25}{4}-\frac{1}{2}\right)\)

\(=2\left[\left(x+\frac{5}{2}\right)^2-\frac{27}{4}\right]\)

\(=2\left(x+\frac{5}{2}\right)^2\)\(=\frac{27}{2}\)> hoặc = \(\frac{-27}{2}\)\(=-13,5\)

Dấu bằng xảy ra  \(\Leftrightarrow\)\(x+\frac{5}{2}=0\)

                                    \(x=\frac{-5}{2}=-2,5\)

Vậy GTLN của A bằng -13,5 khi x = -2,5

b)  \(B=3x-2x^2\)

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

\(=-2\left[\left(x-\frac{3}{4}\right)^2-\frac{9}{16}\right]\)

\(=-2\left(x-0,75\right)^2\)\(+\)\(\frac{9}{8}\)< hoặc = \(\frac{9}{8}\)\(=\)\(1,125\)

Dấu bằng xảy ra  \(\Leftrightarrow\)\(x-0,75=0\)

                                    \(x=0,75\)

Vậy GTLN của B bằng 1,125 khi x = 0,75

kjkkm

=-2,5 đó

Ta có:
\(\left|3x-1\right|\ge0\)

Dấu "=" xảy ra \(\Leftrightarrow x=\dfrac{1}{3}\)

\(\left(2y-1\right)^2\ge0\)

Dấu "=" xảy ra \(\Leftrightarrow y=\dfrac{1}{2}\)

\(\Rightarrow\left|3x-1\right|+\left(2y-1\right)^2+2021\ge0\)

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

Vậy \(A_{min}=2021\Leftrightarrow\left\{{}\begin{matrix}x=\dfrac{1}{3}\\y=\dfrac{1}{2}\end{matrix}\right.\)

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