题目内容
已知向量
=(m,-1),
=(
,
),
(Ⅰ)若
∥
,求实数m的值;
(Ⅱ)若
⊥
,,求实数m的值;
(Ⅲ)若
⊥
,且存在不等于零的实数k,t使得[
+(t2-3)
]•(-k
+t
)=0,试求
的最小值.
. |
a |
. |
b |
1 |
2 |
| ||
2 |
(Ⅰ)若
a |
b |
(Ⅱ)若
a |
b |
(Ⅲ)若
a |
b |
a |
b |
a |
b |
k+t 2 |
t |
(1)∵
=(m,-1),
=(
,
),且
∥
,
∴m
-
.(-1)=0,∴m=-
.
(2)∵
=(m,-1),
=(
,
),且
⊥
,
∴
•
=0,m•
+(-1)
=0,∴m=
.
(3)∵
⊥
,∴
•
=0.
由条件可得|
|=
= 2,|b| =
=1,[
+(t2-3)
]•(-k
+t
)=0,
即:-k
2+(t2-3)t
2=0,即-k|
|2+(t2-3)t|
|2=0,即-4k+(t2-3)t=0.
∴k=
,由
=
=
(t2+4t-3)=
(t+2) 2-
,
可得当t=-2时,
有最小值-
.
. |
a |
. |
b |
1 |
2 |
| ||
2 |
a |
b |
∴m
| ||
2 |
1 |
2 |
| ||
3 |
(2)∵
. |
a |
. |
b |
1 |
2 |
| ||
2 |
a |
b |
∴
. |
a |
. |
b |
1 |
2 |
| ||
2 |
3 |
(3)∵
. |
a |
. |
b |
. |
a |
b |
由条件可得|
a |
|
|
a |
b |
a |
b |
即:-k
a |
b |
a |
b |
∴k=
(t2-3)t |
4 |
k+t2 |
t |
t3-3t+4t2 |
t |
1 |
4 |
1 |
4 |
7 |
4 |
可得当t=-2时,
k+t2 |
t |
7 |
4 |
练习册系列答案
相关题目