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问题 选择题
在空间四边形OABC中，
OA
=
a
，
OB
=
b
，
OC
=
c
，点M在线段OA上，且OM=2MA，N为BC的中点，则
MN
等于（　　）
A．
1
2
a
-
2
3
b
+
1
2
c
B．-
2
3
a
+
1
2
b
+
1
2
c
C．
1
2
a
+
1
2
b
-
2
3
c
D．
2
3
a
+
2
3
b
-
1
2
c
答案

因为空间四边形OABC如图，

OA
=
a
，
OB
=
b
，
OC
=
c
，

点M在线段OA上，且OM=2MA，N为BC的中点，

所以

ON
=
1
2
c
+
1
2
b
．

所以

MN
=
ON
+
MO
=-
2
3
a
+
1
2
b
+
1
2
c
．

故选B．

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