S bang 2 mu 2 cong 4 mu 2 cong 6 mu 2 cong.......cong 20 mu 2

tong S =3 mu 0 cong 3mu 2cong 3mu 4 cong 3 mu 6 cong 3 mu 8 cong .......... cong 3 mu 2006 cong 3 mu 2008 tan cung bang chu so nao . vi sao

chung to rang B = 1/2mu 2 cong 1/3 mu 2 cong 1/4 mu 2 cong 1/5 mu 2 cong 1/6 mu 2cong 1/7 mu 2 cong 1/8 mu2 nho hon 1

cmr A=1 cong 5 cong 5 mu 2 cong 5 mu 3 cong ....cong 5 mu 21 chia het cho 6

 tim so tu nhien x 

3 mu x cong 3 mu x cong 1 cong 3 mu x cong 2 = 1053

1) Tính:

a) A = 7 mu 2 phan 2 nhan 9 cong 7 mu 2 phan 9 nhan 16 cong 7 mu 2 phan 16 nhan 23 cong van van cong 7 mu 2 phan 65 nhan 72

Chung to 4 cong 4 mu 2 cong 4 mu ba +...+ 4 mu 100 chia het cho 20

2 mu m cong 2 mu n bang 2 mu m cong n tim m va n