... 20 11 20 12 20 12 20 12
20 12 S C C C C C C = + + + + + + + L L
0 ,25
( ) ( )
20 11 20 12
20 11 20 12 20 12
20 12 1 1 1 1 20 12 2 2 1007 2 S = + + + = × + = ×
0 ,25
Vậy
20 12
1007 2 S = ×
0 ,25
Câu VI B ... )
1
20 12 20 12 20 12 20 12 2011 20 12
20 12!
1 20 12
! 20 12 !
k k k k k k
k C kC C k C C C
k k
-
+ = + = + = +
-
với 0,1 ,2, ,20 12...
... 2 3 2
2 2 2 2 2
1 1
n
n
n n n n n
C C C C C+ = + + + + +
( )
2
0 1 2 3 2
2 2 2 2 2
1 1
n
n
n n n n n
C C C C C− = − + − + +
( )
1 3 5 2 1 2
2 2 2 2
1 3 5 2 1 2 1
2 2 2 2
2 2
2
n n
n n n ... 4 2 2 1
2 1 2 1 2 1 2 1 2 1
2 3 4 2 1
n n
n n n n n
C xC x C x C n x C
+
+ + + + +
+ + + + + +
0.5
VII
Thay x= -2 vào ta được:
( )
1 2 2...
... + + + + +
2
2
x x
y z x y z
= −
+ + +
. T
ươ
ng t
ự
, c
ộ
ng l
ạ
i ta
đượ
c
VT (1)
2 2 2
1
2 2 2
x y z
y z z x x y
≥ + + −
+ + +
0 ,25
2 2 2 2
2( )
2 1 1
2 2 2 3( )
x y z x y z
xy ... + = + + ≤ + + + +
2 2
2
2
1 1 2 2
( )( 2 ) ( )( 2 )
( ) ( )
x xy x xy
x y z y z x y z y z
y zx z y zx z
+ +
⇒
≥ ⇔ ≥
+ + + + + +
+ + + +
0 ,25
2
2
1 2 1 2 2 2...