Ta có \(\overline {abcd} = 100.\overline {ab} + \overline {cd} = 4.25.\overline {ab} + \overline {cd} \)
Mà \(4.25.\overline {ab} \;⋮\; 4,\) nên ta có:
\(a)\) Nếu \(\overline {cd} \; ⋮\; 4\) thì \(\overline {abcd} \;⋮\; 4.\)
\(b)\) Nếu \(\overline {abcd} \; ⋮\; 4\) thì \((4.25.\overline {ab} + \overline {cd} )\; ⋮\; 4\) nên \(\overline {cd} \; ⋮ \; 4.\)
