Tìm các giá trị m để  bất phương trình sau đây có nghiệm :
1)32x+1(m+3)3x2(m+3)<0(1)2)4x(2m+1)2x+m2+m0(2)
1)  
TXĐ: R.
Đặt t=3x,t>0
(1)  có nghiệm PTf(t)=3t2(m+3)t2(m+3)<0
                                                  có nghiệm t>0
Doa=3>0,Δ=(m+3)(m+27)
Δ0f(t)0mR(1) vô nghiệm
Δ>0[m<27m>3f(t)<0t1<t<t2
Ta có P=t1t2=23(m+3),S=t1+t2=m+33
- Nếu m<27 ta có {P<0S>0t1<t2<0
(2) không có nghiệm dương nên (1) vô nghiệm.
- Nếu m>3  ta có {P<0S>0t1<0<t2
f(t)<0 có nghiệm dương là 0<t1,t2, do đó (1) có nghiệm.
Vậy với m>3 thì (1) có nghiệm.

2) 
Đặt t=2x,t>0
(1)  có nghiệm PTf(t)=t2(2m+1)t+(m2+m)0
                                                  có nghiệm t>0
Doa=1>0,Δ=1
Δ>0m.
t=(2m+1)±Δ2
[t1=mt2=m+1
Đề bài thỏa mãn khi t1t2 dương.
Suy ra m>1.
ĐS : m>1,(2) có nghiệm.

Thẻ

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