问题 填空题
在▱ABCD中,
AB
=a,
AD
=b,
AN
=3
NC
,M为BC的中点,则
MN
=______(用a,b表示).
答案

AN
=3
NC
,得4
AN
=3
AC
=3(
a
+
b
),

AN
=
3
4
a
+
b
),

又∵

AM
=
a
+
1
2
b

MN
=
AN
-
AM
=
3
4
a
+
b
)-(
a
+
1
2
b
)=-
1
4
a
+
1
4
b

故答案为:-

1
4
a
+
1
4
b

补全对话,情景问答
选择题