题目内容

已知|
a
|=4
|
b
|=2
,且
a
b
夹角为120°,求
(1)|
a
+
b
|

(2)
a
a
+
b
的夹角.
(1)∵|
a
|=4
|
b
|=2
,且
a
b
夹角为120°
a
b
=|
a
||
b
|cos120°
=4×2×(-
1
2
)
=-4
|
a
+
b
|=
(
a
+
b
)2
=
a
2
+2
a
b
+
b
2
=
16+4-8
=2
3

(2)设
a
a
+
b
的夹角θ
则cosθ=
a
•(
a
+
b
)
|
a
||
a
+
b
|
=
a
2
+
a
b
4×2
3
=
16-4
8
3
=
3
2

∵0≤θ≤π
θ=
π
6
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