Seasider Posted March 12, 2014 Share Posted March 12, 2014 Other than being a nifty math trick, do you ever use this? We just hit it in LOF Jellybeans. I'm frankly fascinated - this is completely new to me. I'm curious if others of you parents were taught this as young students. Quote Link to comment Share on other sites More sharing options...
kiana Posted March 12, 2014 Share Posted March 12, 2014 It is not necessary. It used to be very necessary for people who were doing accounting without a calculator or adding machine and is still useful for checking if you do a lot of adding of numbers without a calculator. Quote Link to comment Share on other sites More sharing options...
Ravin Posted March 13, 2014 Share Posted March 13, 2014 I used it as a kid in division, to see if things were divisible by 3 (I.e. both numerator and denominator). Quote Link to comment Share on other sites More sharing options...
kiana Posted March 13, 2014 Share Posted March 13, 2014 Since we're on the topic, here are some other cool divisibility rules. If you add up the digits of a number and the sum is divisible by three, the number is divisible by 3. If you add up the digits of a number and the sum is divisible by nine, the number is divisible by 9. If you add and subtract the digits in an alternating manner, and the sum is divisible by 11, the number is divisible by 11. An example of the last: Consider 143418. The alternating sum is 1-4+3-4+1-8 = 5-16 = -11, which is divisible by 11. Therefore 143418 is divisible by 11. Quote Link to comment Share on other sites More sharing options...
IsabelC Posted March 13, 2014 Share Posted March 13, 2014 Never even heard of it until LoF! Quote Link to comment Share on other sites More sharing options...
Missouri Okie Posted March 13, 2014 Share Posted March 13, 2014 Right Start D uses it. There is a lesson on it, and the student is subsequently asked to check their work using this rule (often not always). I had not heard of it previously. Quote Link to comment Share on other sites More sharing options...
PuddleJumper1 Posted March 13, 2014 Share Posted March 13, 2014 A Beka teaches it also (3rd or 4th I think - we didn't use AB beyond that). We honestly tried it once and then skipped it all from there out. Ds got it and thought it was a waste. Dd was confused by it and we didn't think it was a hill we needed to die on so we ignored it. I have never heard of anyone (outside of possibly accounting) that uses it or for that matter has even heard of it. Quote Link to comment Share on other sites More sharing options...
In The Great White North Posted March 13, 2014 Share Posted March 13, 2014 Never heard of it. And I'm old enough to have been issued a slide rule in college (ie. I did lots of adding without a calculator or adding machine.) Quote Link to comment Share on other sites More sharing options...
Clear Creek Posted March 13, 2014 Share Posted March 13, 2014 It is taught in Rod & Staff. If I didn't always have a calculator handy (on my phone) it might be helpful, but since I do, it is just a cool trick to know. Quote Link to comment Share on other sites More sharing options...
Walking-Iris Posted March 13, 2014 Share Posted March 13, 2014 Okay maybe dumb question, but what is casting out 9s? I don't recognize that phrase, so I'm not sure if we have done it or not. Can someone give an example? (I don't use LOF, Abeka, or RS by the way). Quote Link to comment Share on other sites More sharing options...
In The Great White North Posted March 13, 2014 Share Posted March 13, 2014 Okay maybe dumb question, but what is casting out 9s? I don't recognize that phrase, so I'm not sure if we have done it or not. Can someone give an example? (I don't use LOF, Abeka, or RS by the way). I had to Google it. http://en.wikipedia.org/wiki/Casting_out_nines Quote Link to comment Share on other sites More sharing options...
Walking-Iris Posted March 13, 2014 Share Posted March 13, 2014 I had to Google it. http://en.wikipedia.org/wiki/Casting_out_nines I have never heard of that. And it begs the question...why? What would be the point? Other than a random puzzle type activity. Quote Link to comment Share on other sites More sharing options...
kiana Posted March 13, 2014 Share Posted March 13, 2014 I have never heard of that. And it begs the question...why? What would be the point? Other than a random puzzle type activity. It is a way to check for arithmetical errors that is much, much quicker than redoing the calculation. With as much technological assistance as we have now, it is of little use, but if I were adding pages of numbers in an accounts book and did not have a calculator, it would be an extremely good idea to use this for a check. Quote Link to comment Share on other sites More sharing options...
Tenaj Posted March 13, 2014 Share Posted March 13, 2014 CLE Math teaches it in the fifth grade book for sure, maybe in the fourth, also . . .I can't remember when it started but they don't call it casting out nines, they call it digit sums. My 10yods is doing it in his sixth grade R&S right now and I'll tell you, it is a great, quick way for him to check his multiplication and division problems and his careless errors have gone way down! He also just thinks it's a cool thing :) Quote Link to comment Share on other sites More sharing options...
athomeontheprairie Posted March 13, 2014 Share Posted March 13, 2014 Do I think it is necessary? No. do I think it is a way to show how awesome math can be? Yes. Quote Link to comment Share on other sites More sharing options...
Aloha2U Posted March 15, 2014 Share Posted March 15, 2014 It is a way to check for arithmetical errors that is much, much quicker than redoing the calculation. With as much technological assistance as we have now, it is of little use, but if I were adding pages of numbers in an accounts book and did not have a calculator, it would be an extremely good idea to use this for a check. This, in bold text. I usually don't allow my little man to use a calculator during our math studies, so utilizing the "check numbers" method comes in handy when working through multi-step problem solving—which includes addition, subtraction, multiplication, division, etc.—during geometric, algebraic, or word problems. It prevents much frustration and ultimately saves time. Quote Link to comment Share on other sites More sharing options...
Seasider Posted March 16, 2014 Author Share Posted March 16, 2014 Thanks, all, for your info and some additional nifty tricks. I have never been truly math-smart, so I never would have said it was my favorite subject. However, tidbits like this are fascinating to me. Makes me appreciate how black and white and yet surprising math can be, especially in a subjective world. Quote Link to comment Share on other sites More sharing options...
Recommended Posts
Join the conversation
You can post now and register later. If you have an account, sign in now to post with your account.