Bài 1: \(C=3m^2-6m=3m^2-6m+3-3\)
\(=3\left(m^2-2m+1\right)-3\)
\(=3\left(m-1\right)^2-3\ge-3\forall m\)
Vậy: Min C = -3 tại m = 1
Bài 2: \(a,\left(x+3\right)^2-\left(x-3\right)\left(x+3\right)=5\)
\(\Leftrightarrow x^2+6x+9-x^2+9=5\)
\(\Leftrightarrow6x=-13\)
\(\Leftrightarrow x=-\frac{13}{6}\)