Để $PT(1)$ có 2 nghiệm $x_1,x_2$ cùng dấu
$⇔\left \{ {{Δ>0} \atop {x_1.x_2>0}} \right.⇔$ $\left \{ {{1^2-4.2(-m+4)>0} \atop {\frac {-m+4}2>0}} \right.$
$⇔\left \{ {{1-8(-m+4)>0} \atop {-m+4>0}} \right.$ $⇔\left \{ {{1+8m-32>0} \atop {m<4}} \right.$
$⇔\left \{ {{8m>31} \atop {m<4}} \right.$ $⇔\left \{ {{m>\frac{31}8} \atop {x=4}} \right.⇔\frac{31}8<m<4$
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