问题 解答题
已知点O是△ABC内的一点,∠AOB=150°,∠BOC=90°,
OA
=
a
OB
=
b
OC
=
c
,|
a
|=2,|
b
|=1,|
c
|=3.
(1)设实数t满足(
AB
-t
OC
)•
OC
=0,求t的值;
(2)试用
a
b
表示
c
答案

(1)∵(

AB
-t
OC
)•
OC
=0,

∴(

OB
-
OA
-t
OC
)•
OC
=0

∵∠AOB=150°,∠BOC=90°

∴∠AOC=120°,

OA
=
a
OB
=
b
OC
=
c
,|
a
|=2,|
b
|=1,|
c
|=3

∴-3-9t=0

∴t=

1
3

(2)设

c
=m
a
+n
b
,则
c
c
=m
a
c
+n
b
c
c
b
=m
a
b
+n
b
2

9=-3m
0=m•1•2•(-
3
2
)+n

∴m=-3,n=3

3

c
=-3
a
+3
3
b

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