题目内容

已知|
a
|=1,|
b
|=2,<
a
b
>=60°
,则|2
a
-
b
|
=(  )
A.2B.4C.2
2
D.8
|
a
|=1,|
b
|=2,<
a
b
>=60°

a
b
=|
a
|•|
b
|•cos60°
=1×2×
1
2
=1,
因此(2
a
-
b
)2
=4|
a
|2-4
a
b
+|
b
|2=4×12-4×1+22=4,
|2
a
-
b
|
=
(2
a
-
b
)
2
=2(舍负).
故选:A
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