I was playing with additions and it seems that 9 cancels itself out (Sorry, nobody thought me this in elementary algebra, or I forgot) when continuously adding all the digits in a number in order to reach one final one-digit number.<p>Example:<p>92. 9+2=11 -&gt; 1+1=2. So just remove the 9 and you have 2<p>989. 17+9 -&gt; 8 + 9 =&gt; 1 + 7 = 8. So just leave out the 9s.<p>94968594892 -&gt; 9+4+9+6+8+5+9+4+8+9+2 = 73 -&gt; 7+3 = 10 -&gt; 1+0 =1<p>Remove the 9s<p>4+6+8+5+4+8+2 = 37 -&gt; 3+7 = 1<p>So why do the smallest one digit number (0) and the largest one digit number (9) both cancel out in such additions?
Ask HN: Mathematicians, Why 9 cancels out in an addition of digits?