CÂU 6(3 ĐIỂM). CHO HÌNH CHÓP S ABCD. , ĐÁY ABCD LÀ HÌNH CHỮ...

1,0

   

 



















+

MN MB BC CN

1

(AB KB) BC

1

BA

2

2



 

+

1

.

MN

KB BC



2



+ Do đó:

 

  









4BM.MN (BA BK).(KB 2BC)

 

   

 







= BA.KB 2BA.BC BK.KB 2BK.BC

   

 





= BA.KB BK.KB 2BK.BC

 

.(BA BK 2.BC)

  







= KB

    







= KB.(BA BC BK BC)

  

   









= KB.(CA CK) KB.CA KB.CK 0

Vậy:

BK



MN

.