Question

已知三點A(2,3),B(-1,-1),C(6,k),其中k為常數.若||=||,則與的夾角為(    )

A.arccos(-)                   B.或arccos

C.arccos                     D.或π-arccos

Answer 1

解析: =(-3,-4), =(4,k-3),

由||=||5=,求得k=0或k=6.

當k=0時, =(4,-3),則cos〈,〉=0,

所以夾角為;

當k=6時, =(4,3),則cos〈,〉=-,

又因夾角范圍為［0,π］,則〈,〉=π-arccos,故選D.

答案:D