If you multiply 9 by any whole number (except zero), and repeatedly add the digits of the answer until it's just one digit, you will end up with nine.

Dividing the digits by 9 will always give us the same digit repeated to infinity, examples:1 / 9 = 0.11111 3 / 9 = 0.33333

The digit sum of all angles of any polygon will always equal 9.

If a multi-digit number is a multiple of nine, all other numbers formed by rearrangement of its digits also will be multiples of nine.

If one or more zeroes are added after any place in a number that is a multiple of nine, the resultant number will remain a multiple of nine.

If one or more nines or are introduced in any place in a number that is a multiple of nine, the resultant number also will be a multiple of nine.

If any number is reversed and the smaller of the two numbers is subtracted from the larger, then the resulting number will always be a multiple of nine.

If any two digits of a multi-digit number are interchanged and the smaller of the two numbers is subtracted from the larger, the result will always be a multiple of nine.