... )6 6ABCDtV AB AC AD tt L = ộ = = - = ị ờ = - ở uuur uuur uuur ( )30 0D 0 ,25 Gi smtcungoitiptdin ABCDl 2 2 2 ( ) : 2 2 2 0S x y z ax by cz d + + + + + + = ( ) 2 2 2 0a b c d + + - ... A, B,C,Dnờntacúh 2 2 2 3 2 4 5 2 16 9a b c d a c d b d a d + + + = - ỡ ù - + + = - ù ớ - + = - ù ù + = - ợ0 ,25 Giihtrờntac 2, 2, 3, 3a b c d = - = = - =Vyphngtrỡnhmtcu 2 2 2 ( ) ... -0 ,25 Gi A lgiaocatimcnngvi ( ) D ,suyra6 (2 2) 2 Aa + -Blgiaocatimcnngangvi ( ) D ,suyra (2 22) B a -0 ,25 bKhiú 2 236 (2 4)( 2) AB aa = - + -,theobiratacúphngtrỡnh 2 2364( 2) 40(...