\(A=\frac{1}{4}\left(x+2\right)^2-2\ge-2\)
\(A_{min}=-2\) khi \(x=-2\)
Với 2 câu B, C cần kiến thức lớp 9 để làm:
\(Bx^2+2Bx+3B=x^2-2x+2\)
\(\Leftrightarrow\left(B-1\right)x^2+2\left(B+1\right)x+3B-2=0\)
\(\Delta'=\left(B+1\right)^2-\left(B-1\right)\left(3B-2\right)\ge0\)
\(\Leftrightarrow2B^2-7B+1\le0\Rightarrow\frac{7-\sqrt{41}}{4}\le B\le\frac{7+\sqrt{41}}{4}\)
\(B_{min}=\frac{7-\sqrt{41}}{4}\) khi \(x=\frac{\sqrt{41}-1}{4}\)
\(2Cx^2+4Cx+9C=x^2-2x-1\)
\(\Leftrightarrow\left(2C-1\right)x^2+2\left(2C+1\right)x+9C+1=0\)
\(\Delta'=\left(2C+1\right)^2-\left(2C-1\right)\left(9C+1\right)\ge0\)
\(\Leftrightarrow14C^2-11C-2\le0\Rightarrow\frac{11-\sqrt{233}}{28}\le C\le\frac{11+\sqrt{233}}{28}\)
\(C_{min}=\frac{11-\sqrt{233}}{28}\) khi \(x=\frac{\sqrt{233}-11}{8}\)
