đặt gtnn gtln nếu có m=x ²+5x-2

$$\eqalign{
& m = {x^2} + 5x – 2 \cr
& m = {x^2} + 2.x.{5 \over 2} + {{25} \over 4} – {{25} \over 4} – 2 \cr
& m = {\left( {x + {5 \over 2}} \right)^2} – {{33} \over 4} \cr
& {\left( {x + {5 \over 2}} \right)^2} \ge 0\,\,\forall x \Rightarrow {\left( {x + {5 \over 2}} \right)^2} – {{33} \over 4} \ge – {{33} \over 4} \cr
& \Rightarrow m \ge – {{33} \over 4} \cr
& \Rightarrow GTNN\,\,cua\,\,m\,\,la\,\, – {{33} \over 4} \cr
& Dau\,\, = \,\,xay\,\,ra \Leftrightarrow x = – {5 \over 2} \cr} $$