问题
选择题
三角形ABC的外接圆圆心为O且半径为1，若3O
|
答案
∵3
+4OA
+5OB
=OC 0
∴3
+4OA
=-5OB OC
∴9
2+24OA
•OA
+16OB
2=25OB
2OC
∵A，B，C在圆上
∴OA=OB=OC=1
∴
•OA
=0OB
∴
•OC
=-AB
(31 5
+4OA
)• (OB
-OB
)OA
=-
(31 5
•OA
+4OB
2-3OB
2-4OA
•OA
)OB
=-1 5
故选B．