1) Mot so nguyen to chia cho 30 duoc so du la r.Tim r,biet r khong phai la so nguyen to.
2) Chung minh rang:
a/ So 17 khong viet duoc tong 3 hop so khong bang nhau.
b/Moi so le lon hon 17 deu viet duoc duoi dang 3 hop so khac nhau.
chung minh moi so le > 17 deu viet duoc duoi dang tong cua 3 hop so
chung minh rang moi so nguyen to lon hon 2 deu viet duoc duoi dang 4n+1 hoac 4n+3
viet 108 duoi dang tong cac so tu nhien lien tiep lon hon 0
chung minh rang voi moi so tu nhien n # 0 thi:
a moi so nguyen to lon hon 2 deu co dang 4n _+1
b moi so nguyen t lon hon 3 deu co dang 6n +_1
Viet phan so 11/12 duoi dang tong cua hai phan so co mau khac nhau
chung minh rang moi so lon hon 3 deu co dang 6^n+_1
tim 2 so biet tong chung bang 176 moi so deu co 2chu so khac nhau . Si nay la so viet theo thu tu nguoc lai
viet moi so sau deu viet duoi dang cua 2 so tu nhien lien tiep a.444...4(100cso 4)222...2(100cso2)
