问题
解答题
已知|
(1)若
(2)若(2
|
答案
(1)∵|
|=4,|a
|=3且b
,a
夹角为60°b
∴
•a
=|b
|•|a
|•cos60°=6b
∴(
+2a
)•(b
-3a
)=b
2-a
•a
-6b
2=-44b
(2)(2
-3a
)•(2b
+a
)=4b
2-4a
•a
-3b
2=37-4b
•a
=61b
cosθ=
=-
•a b |
|•|a
|b 1 2
又0°≤θ≤180°
θ=120°