问题 填空题
若向量
AB
=(3,-1),
n
=(2,1)
,且
AC
n
=7
,则
n
BC
=______.
答案

AC
=
AB
+
BC

AC
n
=7

(

AB
+
BC
)•
n
=7

AB
n
+
BC
n
=7

AB
=(3,-1),
n
=(2,1)

AB
n
=5

n
BC
=7-5=2

故答案为:2

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