X X X X 1 2 1 22 22( )X X X X X X X X 1 2 1 2 1...
1 .x x x x
1
2
1
2
2
2
2(1 . )x x x x x x x x 1
2
1
2
1
2
1
2
=1
2
1
2
2
2
2(1 . )x x x x x x x x 1
2
1
2
1
2
1
2
=