问题:
已知|
a
|=4,|
b
|=5,|
a
+
b
|=
21
,求:①
a
•
b
②(2
a
-
b
)•(
a
+3
b
)
已知|
|=4,|
|=5,|
+
|=
,求:①
•
②(2
-
)•(
+3
)
a |
b |
a |
b |
21 |
a |
b |
a |
b |
a |
b |
参考答案: