A) CK 1N CKN CKN 1 B) CM 1N CM 1N 2CMN CM 1N 22N D) A5NA6N(N4) 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