4, Ta có : ababab : ab = 10101
=> ababab ⋮ 10101
5, Ta có : abab - baba
= (a000 + b00 + a0 + b) - (b000 + a00 + b0 + a)
= a000 + b00 + a0 + b - b000 - a00 - b0 - a
= (a000 - a00 + a0 - a) - (b000 - b00 + b0 - b)
= 909 x a - 909 x b
= 909 x (a - b)
=> (abab - baba) : 9
