Question

Prove that if you keep adding the digits of any number and get the answer 3,6 or 9 then the number is divisible by 3. I used to know this proof but my brain circuits cant reach it anymore!

Answer 1

the rule of 9 right?

Answer 2

15 = 6 which divides by 3

i know that the factors of 9 add up to 9; might be a correlary to that :)

Answer 3

9*11 = 99 tho so it dont count :)

Answer 4

yes

Answer 5

so get an odd number you have to add by at least one odd right maybe?

Answer 6

but 6 is even lol

Answer 7

i cant recall if i ever knew this :)

Answer 8

i did but its completely gone

Answer 9

if u take a 3 digit number abc then if we can prove that a + b + c = n/3 where n is an positive integer that would prove it true for 3 dgts

Answer 10

a number N can be written a0 + 10a1 +10^2a2 etc.
10 = 1 mod 9 so 10^r = 1 mod 9
so N = a0 + a1 + a2... mod9 and relsult follows (ie divisible by 3 iff sum of digits is).

Answer 11

Jimmy I have a question I need to find the domain of a rational function
f(x)=3x-8
__________
4x+20