Digits Of Multiples Of Nine Sum To Nine In My Research On Digit 9

Digits 0,1,2,3,4,5,6,7,8,9 are the first ten digits were introduced in Mathematics.

These ten digits are used in everyday numbers.
DIGIT 9 is the only digit number that can be multiply with any other digit/number except zero,the results,either added or subtract it,the digit number 9 is still appear

For instance,
FIRST DIGIT
1x9=9 (9 appear)
2x9=18 (Add 1 and 8 together, we still get 9)
3x9=27 (Add 2 and 7 together, we still get 9)
4x9=36 (Add 3 and 6 together,we still get 9)
TWO DIGITS
21x9=189 (9 appear)
34x9=306 (Add 3,0 and 6 together,we still get 9)
58x9=522 (Add 5,2 and 2 together,we still get 9)
THREE DIGITS
123x9=1107 (add 1,1,0 and 7 together,we still get 9)

342x9=3078 (add 3,0,7 and 8 together 18 then add 1 and 8 again 9 we still appear)

706x9=6354 (add 6,3,5and 4 together 18 then add 1 and 8 again 9 we still appear)

DECIMAL NUMBER
0.5x9=4.5 (add 4 and 5,we still get 9)
0.31x9=2.79 (add 2,7 and 9 together 18 then add 1 and 8 again. We still get 9)

Test it with any numerals, 9 will still appear.