问题
选择题
设P为等边△ABC所在平面内的一点，满足
|
答案
因为
•PA
=（PB
+PC
）•（CA
+PC
）CB
=
•PC
+PC
•（PC
+CA
）+CB
•CA CB
=（
+2CB
）•（CA
+2CB
）-（CA
+2CB
）•（CA
+CA
）+CB
•CA CB
=2
2+2CA
•CA CB
=2×12+2×1×1×1 2
=3．
故选 B．
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