4X X X X X XX X X XX X X X S S P− = = = ± = ± 1 2 12 22 ( 1 2)(...
4 . 4x x x x x xx x x xx x x x S S P− = = = ± = ±
1
2
1
2
2
2
(
1
2
)(
1
2
) (
1
2
) (
1
2
)
2
1 2
2
x x x x x x x x P2
1
1 2
1 2
1 2
x1
3
−x2
3
=(
x1
−x2
) (
x1
2
+x x1
.2
+x2
2
)
=(
x1
−x2
) (
x1
+x2
)
2
−x x1
.2
.(
x1
x2
2
4x x1 2
)
(
x1
x2
)
2
x x1
.2
(
S2
4P)
S2
P( )
= ± + − + − = ± − − . x1
4
−x2
4
=( ) ( ) (
x1
2
2
− x2
2
2
= x1
2
+x2
2
)(
x1
2
−x2
2
) (
= ± S2
−2P) (
S. S2
−4P