Bài 2:
1: \(=x^2+6x+9+4=\left(x+3\right)^2+4>=4\)
Dấu '=' xảy ra khi x=-3
2: \(=-\left(x^2-4x-1\right)\)
\(=-\left(x^2-4x+4-5\right)\)
\(=-\left(x-2\right)^2+5< =5\)
Dấu '=' xảy ra khi x=2
3: \(=9x^2+12x+4-9=\left(3x+2\right)^2-9>=-9\)
Dấu '=' xảy ra khi x=-2/3