How can you quickly check whether a number is exactly divisible by 9?
According to mathematicsnotebook:
A number is divisible by 9 if the sum of its digits is divisible by 9. For large numbers this rule can be applied again to the result. In addition, the final iteration will result in a 9.
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What are your thoughts on this subject?
45 Comments
Louella Wilson
I've always thought it a useless fact. Interesting, but useless. I can divide by 9 as quickly, or quicker, than adding digits before dividing that sum by 9.
Georgina Morrison
SUM is the answer to an addition question and PRODUCT is the answer to a multiplication questionHenry Ford 1889,
Henry Ford 1889
When I was at school, sum and product were one and the same.
Chris Brown
Or if the separate digits add up to 9. 9x2=18; 9x3=27 etc. Also, for any number added to 9, the digits add up to the number added on: 9+1=10; 9+8=17...
LevibatteryCSA (Dave)
I don't use Math any more.
Brian Reynolds
I recently watched a TV program called "The human calculator". The man was incredible. He would stop strangers in the street and after a few questions would tell them their birth dates. He then demonstrated all sorts of calculations with the number nine. In one scene he was in a supermarket and tallied a large customer bill in a matter of seconds. An amazing program and I hope there are more episodes to come.
Nikki Proctor
that is how i taught my girls their 9 x tables. everything adds up to nine (ie 4X9=36, 3+6=9)
Richard Rubright
Actually, the sume of the digits must be exactly 9. If more than 1 digit keep adding until there is only the digit 9.
james parrillo
Joseph A. Robitaille, The question asked another way to tell if a number may be divided by 9 evenly. It is possible to add all the digits in the number and then tell if the sum of the digits is also divisible by 9. Dividing the original number by 9 and adding and dividing that number by 9 both work to tell if the number is evenly divisible by 9. Thank you, Christine, James' wife
james parrillo
docluci, Be well. God bless you. Christine, James' wife
james parrillo
Michael Conway, It is so good you are playing. More of your guesses may be right, and you will learn either way. Thank you, Christine, James' wife
Deb Schlawin
Learned this in middle school as casting out nines! Has come in handy many times.
docluci
GOt this answer
docluci
Used a hint. Am in ICU and doing things is difficult
tbear
tesla alset, got your back, Tesla! 😊 👍
jimsclads
In grade school half a century ago I figured out shortcuts like this by looking at numbers rather than just memorizing times tables, but modern math wants just estimates rather than accuracy.