Ta thấy:33x33=1089(2 c/s 3;1 c/s 1; 1 c/s8)
333x333=110889(3c/s3;2c/s1;2c/s8)
3333x3333=11108889(4c/s3;3c/s1;3c/s8)
=>333...3x333...3(2005 số 3)=111...1(2004 c/s 1)0888...8(2004 c/s 8)9
333...3x333...3=333...32(2005 số 3)
Ta thấy:332=1089(2 c/s 3;1 c/s 1; 1 c/s8)
3332=110889(3c/s3;2c/s1;2c/s8)
33332=11108889(4c/s3;3c/s1;3c/s8)
=>333...32(2005 số 3)=111...1(2004 c/s 1)0888...8(2004 c/s 8)9