91Ó°ÊÓ

Prove that an integer \(n\) is divisible by 5 if and only if its last decimal digit is divisible by 5 .

Prove that an integer \(n\) is divisible by 3 if and only if the sum of its decimal digits is divisible by 3 .

Prove that an integer \(n\) is divisible by 9 if and only if the sum of its decimal digits is divisible by 9 .