题目内容
已知|
|=4,|
|=2,且
与
夹角为120°,求
(1)|
+
|;
(2)
与
+
的夹角.
a |
b |
a |
b |
(1)|
a |
b |
(2)
a |
a |
b |
(1)∵|
|=4,|
|=2,且
与
夹角为120°
∴
•
=|
||
|cos120°=4×2×(-
)=-4
∴|
+
|=
=
=
=2
(2)设
与
+
的夹角θ
则cosθ=
=
=
=
∵0≤θ≤π
∴θ=
a |
b |
a |
b |
∴
a |
b |
a |
b |
1 |
2 |
∴|
a |
b |
(
|
|
16+4-8 |
3 |
(2)设
a |
a |
b |
则cosθ=
| ||||||
|
|
| ||||||
4×2
|
16-4 | ||
8
|
| ||
2 |
∵0≤θ≤π
∴θ=
π |
6 |
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