Ta có: F = 5 + 6x + 9x^2
=> F = (3x)^2 + 2.3x.1 + 1^2 + 4
=> F = (3x+1)^2 +4 \(\ge4\). Dấu "=" xảy ra \(\Leftrightarrow3x+1=0\Rightarrow x=\frac{-1}{3}\)
Vậy: GTNN của F = 4 khi x = -1/3
\(F=5+6x+9x^2\)'
\(F=9x^2+6x+1+4\)
\(F=\left(3x+1\right)^2+4\)
\(Do\left(3x+1\right)^2\ge0\Rightarrow F\ge4\)
Dấu "=" xảy ra khi 3x + 1 =0
<=> 3x = -1
<=> x = -1/3
Vậy Min F = 4 khi x = -1/3