问题:
已知△ABC中,
AB
=
a
,
AC
=
b
,
a
•
b
<0,S △ABC=
15
4
,|
a
|=3,|
b
|=5,则
a
与
b
的夹角是( )
A.30°
B.-150°
C.150°
D.30°或150°
已知△ABC中,
=
,
=
,
•
<0,S △ABC=
,|
|=3,|
|=5,则
与
的夹角是( )
AB |
a |
AC |
b |
a |
b |
15 |
4 |
a |
b |
a |
b |
A.30° | B.-150° | C.150° | D.30°或150° |
参考答案: