问题 解答题
已知向量
a
=(
3
,k)
b
=(0,-1),
c
=(1,
3
)

(Ⅰ)若
a
c
,求k的值;
(Ⅱ)当k=1时,
a
b
c
共线,求λ的值;
(Ⅲ)若|
m
|=
3
|
b
|,且
m
c
的夹角为150°,求|
m
+2
c
|
答案

(Ⅰ)∵

a
c
,∴
a
c
=0
,∴
3
+
3
k=0
,解得k=-1;

(Ⅱ)∵k=1,∴

a
=(
3
,1),又
b
=(0,-1)
,∴
a
b
=(
3
,1-λ)

a
b
c
共线,∴
3
×
3
-(1+λ)=0
,解得λ=2;

(Ⅲ)∵|

b
|=
0+(-1)2
=1,∴|
m
|=
3

m
c
的夹角为150°,|
c
|=
1+(
3
)2
=2.

m
c
=|
m
| |
c
|cos150°=
3
×2×cos150°
=-3,

|

m
+2
c
|=
m
2
+4
m
c
+4
c
2
=
(
3
)2+4×(-3)+4×22
=
7

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