cho A = 1+2+3+4+...+n va B = 2n + 1 ( voi n thuoc N , n lon hon hoac bang 2)   chung minh A va B la hai so nguyen to cung nhau

hay viet cac tap hop sau bang cach khac 

A={ 9;11;13;...;99] 

B={ 12 ; 24 ; 36 ;48}

M= { THANG 4 ; THANG 6 ;THANG 9 ; THANG 11} 

N =  { X THUOC N / X LON HON HOAC BANG 3 , LON HON HOAC BANG 10} 

Q = { X THUOC N / X LON HON 0 ,BE HON 10}

Tap hop A x thuoc N 2 lon hon hoac bang  va be hon hoac bang 78.

(1+1/2)×(1+1/3)×(1+1/4)×....×(1+1/n)

Tim n biet n thuoc N va n lon hon hoac bang 2

Cho day so sau : 1, 2, 3, ..., 200 ( 200 so ). Chon n so ( n lon hon hoac bang 2 ) sao cho co 2 so bat ki trong n so do sao cho tong cua 2 trong n so bat ki do chia het cho 2 voi n lon nhat.

CM 2a+3b=n (voi a,b,n thuoc N ;n lon hon hoac bang 2)

So lien truoc cua so 3m -1 la so nao?( Voi m thuoc N va m lon hon hoac bang 2)

1.     cho 4 stn a lon hon hoac bang b, b lon hon hoac bang c, clon hon hoac bang d.

CM:(a-b)(a-c)(a-d)(b-c)(b-d)(c-d)

2.      CM: co the tim dc 1 stn k sao cho: (1997^k)-1 chia het cho 10^4

3.      tong cac chu so cua 1 so chinh phuong co the bang 1995 dc k?

4.      tong cua 1995 stn khac 0 dung bang 1995. Hoi UCLN cua chung la bao nhieu?