\(A=\left(x-2\right)\left(x+1\right)=x^2-x-2\\ =\left(x-\frac{1}{2}\right)^2-\frac{9}{4}\ge-\frac{9}{4}\)
A min=-9/4 tại x=1/2
\(B=\left(2x-1\right)\left(x-3\right)=2x^2-7x+3\\ =2\left(x^2-\frac{7}{2}x+\frac{3}{2}\right)\\ =2\left(x^2-2x.\frac{7}{4}+\frac{49}{16}-\frac{25}{16}\right)\\ =2\left(x-\frac{7}{4}\right)^2-\frac{25}{8}\ge-\frac{25}{8}\)
B min=-25/8 tại x=7/4