Another s/o - casting out 9s

[ELaurie]

Y'all may already know these, but here are some more cool math tricks that I had forgotten about until I saw them in ds 11's math book:

 

2 Every even number is divisible by 2.

 

3 If the sum of the digits of a number is divisible by 3, the original number is divisible by 3. For example, for the number 57, 5+7=12; 12 is divisible by 3, so 57 will be divisible by 3.

 

4 If the last two digits of the number are divisible by 4, the number will be divisible by 4. For example, 132 is divisible by 4 because 32 is divisible by 4.

 

5 Numbers ending in 0 or 5 are divisible by 5.

 

6 Any number that is divisible by 2 and 3 is divisible by 6.

 

9 If the sum of the digits of the number is divisible by 9, the original number is divisible by 9. Example: for the number 72, since 7+2=9, 72 is divisible by 9.

[kiana]

Another:

 

If the alternating sum of the digits is divisible by 11, then the number is divisible by 11.

 

Example:

 

123456789

 

1-2+3-4+5-6+7-8+9 = 5, which is not divisible by 11, so 123456789 is not divisible by 11.

 

1358024679

 

1-3+5-8+0-2+4-6+7-9 = -11, which is divisible by 11 (even though it's negative), so 1358024679 is divisible by 11.

[ELaurie]

for posting this!

 

:001_smile: