aaa = a x 111 = a x 3 x 37
vậy aaa chia hết cho 37
aaaaaa = a x 111111 = 3003 x 37
vây aaaaaa chia hết cho 37
abcabc = abc x 1001 = abc x 7x11x13
Vậy abc chia hết cho 7 , 11, 13
Ta có:
aaa=a.111=a.3.37:.37=>ĐPCM
aaa aaa=aaa.1001, mà aaa:.37=>aaa aaa:.37=>ĐPCM
abcabc=abc.1001=abc.143.7:.7
=abc.1001=abc.91.11:.11
=abc.1001=abc.77.13:.13
=>ĐPCM
ab+ba=a.10+b+b.10+a=(a.10+a)+(b.10+b)=a.11+b.11=(a+b).11:.11=>ĐPCM
ab-ba=a.10+b-b.10-a=(a.10-a)-(b.10-b)=a.9-b.9=(a-b).9:.9=>ĐPCM