已知a=(1,5,-1),b=(-2,3,5).(1)求a+b与a的夹角的余弦值;(2)若(ka+b)∥(a-3b),求实数
已知a=(1,5,-1),b=(-2,3,5).(1)求a+b与a的夹角的余弦值;(2)若(ka+b)∥(a-3b),求实数k的值;(3)若(ka+b)⊥(a-3b),求...
已知a=(1,5,-1),b=(-2,3,5).(1)求a+b与a的夹角的余弦值;(2)若(ka+b)∥(a-3b),求实数k的值;(3)若(ka+b)⊥(a-3b),求实数k的值.
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(1)
+
=(-1,8,4),
∴|
+
|=
=9,
|
|=
=3
,
(
+
)?
=-1×1+8×5+4×(-1)=35.
∴cos<
+
,
>=
=
=
.
(2)k
+
=(k-2,5k+3,-k+5),
?3
=(7,-4,-16).
∵两向量平行,∴
=
=
,∴k=-
.
(3)∵(k
+
)⊥(
-3
),
∴(k
+
)?(
-3
)=0,即(k-2)-4(5k+3)-16(-k+5)=0,解得k=
.
a |
b |
∴|
a |
b |
(?1)2+82+42 |
|
a |
12+52+(?1)2 |
3 |
(
a |
b |
a |
∴cos<
a |
b |
a |
(
| ||||||
|
|
35 | ||
9×3
|
35
| ||
81 |
(2)k
a |
b |
a |
b |
∵两向量平行,∴
k?2 |
7 |
5k+3 |
?4 |
?k+5 |
?16 |
1 |
3 |
(3)∵(k
a |
b |
a |
b |
∴(k
a |
b |
a |
b |
106 |
3 |
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