A) CK 1N CKN CKN 1 B) CM 1N CM 1N 2CMN CM 1N 22N D) A5NA6N(N4) A2 4N1C) ( 2)
Bài 4. Chứng minh:
a)
C
k 1
n
C
k
n
C
k
n 1
b)
C
m 1
n
C
m 1
n
2C
m
n
C
m 1
n 2
2
n
d)
A
5
n
A
6
n
(n
4) A
2
4
n
1
c)
(
2
)
!
n
!
(
)
e)
A
k
n 1
kA
k 1
n 1
A
k
n