问题 解答题
已知|
a|=
2
|
b|=
3
a
b
的夹角为120°.
(Ⅰ)求
a
b

(Ⅱ)求|
a
+
b
|.
答案

(1)

a
b
=|
a
||
b
|cosθ
=2×3×cos120°=-3

(2)|

a
+
b
|2=
a
2
+2 
a
 •
b
+
b
2

=9+2|

a
||
b
|cos120°+4

=13-6

=7

|

a
+
b
|=
7

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