(9 ) (9 ) ( ) (81 9 2 9 ) ( )A= X Y+ Y Z Z+ − XZ X+ = XY YZ+ + Y + XZ Z− XZ X+ ( ) ( )≥ + + Ữ − + ≥ + + Ữ − + = − + + +

4 . - 115 CAU TRAC NGHIEM CHON LOC
115 CAU TRAC NGHIEM CHON LOC
4 . D. 341 Lời giải:

(

9

) (

9

) ( ) (

81 9

2

9

) ( )

A= x y+ y z z+ − xz x+ = xy yz+ + y + xz z− xz x+ 

( ) ( )

≥ + + ữ − + ≥ + + ữ − + = − + + + . 81 1

2

81 1 18 81