B B( )(2 ) 1 2A B A B A B A B A A A( )(2 ) 2M A A B C A B C B C ( ) 1 A A A(1 )(2 )X X X X 1 2 1 2 =
1
.2
aTa có :b b( )(2 ) 1 2a b a b a b a b a a a( )(2 ) 2
M a a b c a b c b c ( ) 1 a a a(1 )(2 )x x x x 1
2
1
2
=