题目内容
（平面向量）已知|| a |
| b |
| a |
| b |
| a |
| b |
分析：根据|
|=|
|=|
-
|=1可得：|
-
|2=|
|2+|
|2-2
•
=1+1-2×1×1×cosθ=1，即cosθ=
．
由根据|
+2
|2=|
|2+|2
|2+2|
||2
|cosθ，代入即可得到答案．
| a |
| b |
| a |
| b |
| a |
| b |
| a |
| b |
| a |
| b |
| 1 |
| 2 |
由根据|
| a |
| b |
| a |
| b |
| a |
| b |
解答：解：|
|=|
|=|
-
|=1
∵|
-
|2=|
|2+|
|2-2
•
=1+1-2×1×1×cosθ=1
∴cosθ=
|
+2
|2=|
|2+|2
|2+2|
||2
|cosθ=1+4+2=7
|
+2
|=
故答案为：
| a |
| b |
| a |
| b |
∵|
| a |
| b |
| a |
| b |
| a |
| b |
∴cosθ=
| 1 |
| 2 |
|
| a |
| b |
| a |
| b |
| a |
| b |
|
| a |
| b |
| 7 |
故答案为：
| 7 |
点评：本题主要考查向量的数量积运算法则和求模运算．属基础题．
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