1)求证AB=AC
△ABC中,已知(AB+AC)•BC=0
(1)求证:|AB|=|AC|;(2)若|AB|=2,AB•AC=-2,求|BC|
(1)(AB+AC)•BC=0
--->(AC+CB+AC)•BC = 0
--->(2AC-BC)•BC = 0
--->-2AC•BC + BC•BC = 0
--->AC•AC-2AC•BC+BC•BC=AC•AC
--->(AC-BC)•(AC-BC)=AC•AC
--->AB•AB=AC...全部
△ABC中,已知(AB+AC)•BC=0
(1)求证:|AB|=|AC|;(2)若|AB|=2,AB•AC=-2,求|BC|
(1)(AB+AC)•BC=0
--->(AC+CB+AC)•BC = 0
--->(2AC-BC)•BC = 0
--->-2AC•BC + BC•BC = 0
--->AC•AC-2AC•BC+BC•BC=AC•AC
--->(AC-BC)•(AC-BC)=AC•AC
--->AB•AB=AC•AC
--->|AB|²=|AC|²
--->|AB|=|AC|
(2)|AB|=|AC|=2
--->|BC|²=BC•BC=(AC-AB)•(AC-AB)=|AC|²-2AB•BC+|AB|²=12
--->|BC|=2√3。
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