1 |
2 |
=
1 |
2 |
3 |
2 |
令t=log2x,则y=
1 |
2 |
3 |
2 |
1 |
2 |
3 |
2 |
1 |
8 |
∵2≤x≤4,∴1≤t≤2.
当t=
3 |
2 |
1 |
8 |
∴函数的值域是[-
1 |
8 |
(2)令t=log2x,得
1 |
2 |
3 |
2 |
∴m≥
1 |
2 |
1 |
t |
3 |
2 |
设g(t)=
1 |
2 |
1 |
t |
3 |
2 |
∴g(t)=
1 |
2 |
1 |
t |
3 |
2 |
1 |
2 |
2 |
t |
3 |
2 |
∵g(1)=0,g(2)=0,
∴g(t)max=0,∴m≥0.
故m的取值范围是[0,+∞).
1 |
2 |
1 |
2 |
1 |
2 |
3 |
2 |
1 |
2 |
3 |
2 |
1 |
2 |
3 |
2 |
1 |
8 |
3 |
2 |
1 |
8 |
1 |
8 |
1 |
2 |
3 |
2 |
1 |
2 |
1 |
t |
3 |
2 |
1 |
2 |
1 |
t |
3 |
2 |
1 |
2 |
1 |
t |
3 |
2 |
1 |
2 |
2 |
t |
3 |
2 |