\(A=4\left(x-\frac{1}{4}\right)^2+\frac{19}{4}\ge\frac{19}{4}\)
\(A_{min}=\frac{19}{4}\) khi \(x=\frac{1}{4}\)
\(B=-9\left(x-\frac{2}{9}\right)^2-\frac{14}{9}\le-\frac{14}{9}\)
\(B_{max}=-\frac{14}{9}\) khi \(x=\frac{2}{9}\)
\(C=\left(x+1\right)\left(x-6\right)\left(x-2\right)\left(x-3\right)\)
\(C=\left(x^2-5x-6\right)\left(x^2-5x+6\right)\)
\(C=\left(x^2-5x\right)^2-36\ge-36\)
\(C_{min}=-36\) khi \(\left[{}\begin{matrix}x=0\\x=5\end{matrix}\right.\)