ta có abccba+22=100001a+10010b+1100c+22.
Ta thấy 100001a chia hết cho 11 (100001=11x9091)
10010b chia hết cho 11 (10010=11x910)
1100c chia hết cho 11
22 chia hết cho 11
Vậy abccba+22 chia hết cho 11 nên nó là hợp số.
Chung to tong sau la hop so
aabb + 22
chung minh rang tong sau day la hop so abcabc + 22
chung minh rang tong sau la hop so: abcab+22
chung minh rang tong sau la hop so: abcabc+22
Chung minh rang cac tong sau la hop so
a) abcabc + 7
b) abcabc + 22
c) abcabc + 39
chung to rang abcabc+22 la hop so
tong hieu sau la so nguyen to hay hop so
23.27.29+1
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.
tong sau day la so nguyen to hay hop so: 3.5.7 + (2n+1).(2n+3).(2n+5) voi n la so tu nhien
