方程x2+(m-2)x+5-m=0的兩根都大于2,則m的取值范圍是( ) A.(-5,-4] B.(-∞,-4] C.(-∞,-2] D.(-∞,-5)∪(-5,-4]
方程x2+(m-2)x+5-m=0的兩根都大于2,則m的取值范圍是( )
A. (-5,-4]
B. (-∞,-4]
C. (-∞,-2]
D. (-∞,-5)∪(-5,-4]
優(yōu)質(zhì)解答
令f(x)=x
2+(m-2)x+5-m,其對稱軸方程為x=
由已知方程x
2+(m-2)x+5-m=0的兩根都大于2,故有
即
| >2 | 4+2m?4+5?m>0 | (m?2) 2?4(5?m)≥0 |
| |
解得-5<m≤-4
m的取值范圍是(-5,-4]
故應(yīng)選A.
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