问题
解答题
平面内有四个向量
(1)用
(2)若
|
答案
(1)∵
∴
=a
-y x
= 2b
-x y
=x
+a b
=2y
+a b
(2)∵|
|=|a
| =1,b
⊥a b
∴|
|=x
=
2x
=(
+ a
)2b
,|2
|=y
=
2y
=(2
+a
)2b
= 4
2+4a
•a
+b
2b 5
∵
⊥a b
∴
•a
=(b
-y
)•(2x
-x
)y
=3
•x
-y
2-2y
2=0 x
∴3×
×2
cosθ -5-2×2=05
∴cosθ=3 10 10