a) goi d la UCLN cua n ;n+1

ta co n+1-n chia het cho d

1 chia het cho d

suy ra d=1

vi n ;n +1la 2 so nguyen to cung nhau nen n/n+1 la phan so toi gian

b;goi d la UCLN cua n+1 va 2n+3

[n+1 suy ra 2(n+1)

ta co 2n+3-2(n+1) chia het cho d

2n+3-2n+2 chia het cho d

1 chia het cho d

dan den d=1

bai toan duoc chung minh

c;goi d la UCLN cua 21n+4 va14n+3

[21n+4 suy ra 2(21n+4)

[14n+3 suy ra 3(14n+3)

ta co 3(14n+3)-2(21n+4)

42n+9-42n+8

1 chia het cho d

d=1 bai toan duoc chung minh

d)goi d la UCLN cua 2n+3 va 3n+5

[2n+3 suy ra 3(2n+3)

[3n+5 suy ra 2(3n +5)

ta co 2(3n+5)-3(2n+3)

6n+10-6n+9

1 chia het cho d

dan den d bang 1

bai toan duoc chung minh

cac bai tuong tu nhu vay thi hay chung to tu va mau la 2 so nguyen to bang nhau