đặt gtnn gtln nếu có m=x ²+5x-2

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