9x2=18 (1+8=9) 9x3=27(2+7=9) ect.
2007-04-17 12:42:03 · 6 answers · asked by Anonymous in Science & Mathematics ➔ Mathematics
to determine if a number is divisible by 9, add up the digits and see if they add up to 9. if they do, they are divisible by 9.
i feel so trippy.
2007-04-17 12:47:29 · answer #1 · answered by Anonymous · 0⤊ 0⤋
When you add up the digits, you are really subtracting multiples of 9.
For example, 7246
7+2+4+6 = 7*(1000-999) + 2*(100-99) + 4*(10-9) + 6*(1)
= 7*1000 + 2*100 + 4*10 + 6 - 7*999 - 2*99 - 4*9
= 7246 - 9*(7*111+2*11+4)
= 7246 - a multiple of 9
So, if your original number was a multiple of 9, and adding the digits was just subtracting another multiple of 9, then it will result in a multiple of 9.
2007-04-17 19:48:03 · answer #2 · answered by David K 3 · 1⤊ 0⤋
That is exactly how you can test any number to determine if it is divisible by 9.
2007-04-17 19:45:21 · answer #3 · answered by Renaissance Man 5 · 1⤊ 0⤋
you can also use your fingers. hold up your hands so they both face you. 1x9=9 hold down your first finger, count the number left standing.
2x9=18
hold down your second finger. to the left of it you have one standing to the right you have 8
3x9=27
third finger, to the left 2 to the right 7.
------------------------------------------------------------------------------------
also, you can add a number up and see if it's divisible by three.
264
2+6+4=12
12/3=4 so yes, it's divisible by three.
2007-04-17 19:50:03 · answer #4 · answered by Anonymous · 1⤊ 0⤋
Same thing works with 3.
2007-04-17 19:49:11 · answer #5 · answered by Doc Occam 7 · 0⤊ 1⤋
that is crazy!!! thanks for the two points for answering this not-really-a-question question!!!
thanks for the info. too though! that'll be good to know for sometime... I'm sure!
2007-04-17 19:49:17 · answer #6 · answered by Neonkttie 3 · 0⤊ 0⤋