已知正△ABC的边长为1,CP=7CA+3CB,则CP•AB= _ .
问题描述:
已知正△ABC的边长为1,
=7
CP
+3
CA
,则
CB
•
CP
= ___ .
AB
答
由题意可得
•CP
=(7AB
+3CA
)•CB
=7AB
•CA
+3AB
•CB
=7×1×1cos120°+3×1×1cos60°=-AB
+7 2
=-2,3 2
故答案为-2.