c: \(-x^2+2x-2=-\left(x-1\right)^2-1\le-1\forall x\)

\(\Leftrightarrow V\ge-1\forall x\)

Dấu '=' xảy ra khi x=1

a) Giá trị lớn nhất:

\(A=2x-3x^2-4=-3\left(x^2-\frac{2}{3}x+\frac{4}{3}\right)=-3\left[x^2-2.x.\frac{1}{3}+\left(\frac{1}{3}\right)^2+\frac{35}{9}\right]=-3\left(x-\frac{1}{3}^2\right)-\frac{35}{3}\)

Vì \(\left(x-\frac{1}{3}\right)^2\ge0\left(x\in R\right)\)

Nên \(-3\left(x-\frac{1}{3}\right)^2\le0\left(x\in R\right)\)

do đó \(-3\left(x-\frac{1}{3}\right)^2-\frac{35}{3}\le-\frac{35}{3}\left(x\in R\right)\)

Vậy \(Max_A=-\frac{35}{3}\)khi \(x-\frac{1}{3}=0\Rightarrow x=\frac{1}{3}\)

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

Vì \(\left(x+2\right)^2\ge0\left(x\in R\right)\)

nên \(-\left(x+2\right)^2\le0\left(x\in R\right)\)

do đó \(-\left(x+2\right)^2+4\le4\left(x\in R\right)\)

Vậy \(Max_B=4\)khi \(x+2=0\Rightarrow x=-2\)

b) Giá trị nhỏ nhất 

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

Vì \(\left(x-1\right)^2\ge0\left(x\in R\right)\)

nên \(\left(x-1\right)^2-2\ge-2\left(x\in R\right)\)

Vậy \(Min_A=-2\)khi \(x-1=0\Rightarrow x=1\)

\(B=4^2+4x+5=\left(2x\right)^2+2.2x.1+1+4=\left(2x+1\right)^2+4\)

vì \(\left(2x+1\right)^2\ge0\left(x\in R\right)\)

nên \(\left(2x+1\right)^2+4\ge4\left(x\in R\right)\)

Vậy \(Min_B=4\)khi \(2x+1=0\Rightarrow x=-\frac{1}{2}\)

\(A=x^2-4x+10=x^2-4x+4+6=\left(x-2\right)^2+6\ge6\)

Vậy GTNN A là 6 khi x - 2 = 0 <=> x = 2 

\(B=\left(1-x\right)\left(3x-4\right)=3x-4-3x^2+4x=-3x^2+7x-4\)

\(=-3\left(x^2-\frac{7}{3}x+\frac{4}{3}\right)=-3\left(x^2-2.\frac{7}{6}x+\frac{49}{36}-\frac{1}{36}\right)=-3\left(x-\frac{7}{6}\right)^2+\frac{1}{12}\ge\frac{1}{12}\)

\(=3\left(x-\frac{7}{6}\right)^2-\frac{1}{12}\le-\frac{1}{12}\)Vậy GTLN B là -1/12 khi x = 7/6 

\(C=3x^2-9x+5=3\left(x^2-3x+\frac{5}{3}\right)=3\left(x^2-2.\frac{3}{2}x+\frac{9}{4}-\frac{7}{12}\right)\)

\(=3\left(x-\frac{3}{2}\right)^2-\frac{7}{4}\ge-\frac{7}{4}\)Vậy GTNN C là -7/4 khi x = 3/2 

\(D=-2x^2+5x+2=-2\left(x^2-\frac{5}{2}x-1\right)=-2\left(x^2-2.\frac{5}{4}x+\frac{25}{16}-\frac{41}{16}\right)\)

\(=-2\left(x-\frac{5}{4}\right)^2+\frac{21}{8}\le\frac{21}{8}\)Vậy GTLN D là 21/8 khi x = 5/4