( ) ( )( ) ( ) ( ( ) )+ +6 11 12 12 11 11 11 11+

2 .3 2 .3.5. 2.3 2 .3 2 .3 .5 2 .3 1 5 2.6 12

c, Ta có: ( ) ( )

( ) ( ) ( ( ) )

+ +

6 11 12 12 11 11 11 11

+