NotSoObvious Posted November 29, 2011 Share Posted November 29, 2011 For instance, the digit sum of 98 is 8 and the digit sum of 13,132 is 1. I never remember learning these and, quite frankly, I don't really get it or know why my kids need to do it. Can someone convince me otherwise? Quote Link to comment Share on other sites More sharing options...
LNC Posted November 29, 2011 Share Posted November 29, 2011 It is just a quick way to check work. It is actually quite handy. You'll see in future grades if you stick with CLE. Quote Link to comment Share on other sites More sharing options...
Chris in VA Posted November 29, 2011 Share Posted November 29, 2011 What's a digit sum? Never mind--I think I figured it out. Quote Link to comment Share on other sites More sharing options...
boscopup Posted November 29, 2011 Share Posted November 29, 2011 It's really useful in divisibility. For example, if the digit sum is divisible by 3, the number is divisible by 3. Quote Link to comment Share on other sites More sharing options...
RootAnn Posted November 29, 2011 Share Posted November 29, 2011 (edited) What everyone else said + factoring! So, you can see if something is divisible by 3 or 9 if the digit sum is divisible by 3 or 9, respectively. Also, if a number is even and divisible by 3, it is divisible by 6. Good, easy rule of thumb. (Some of this comes in handy when dealing with finding the least common denominator for fractions & reducing fractions.) Edited November 29, 2011 by RootAnn Added the fractions statement Quote Link to comment Share on other sites More sharing options...
NotSoObvious Posted November 29, 2011 Author Share Posted November 29, 2011 So, apparently the way CLE teaches it is not entirely accurate! According to my super computing friend, the examples given are NOT digit sums, but digital roots, which, he says, are not very useful past divisibility tests. Interesting! It was easy enough to teach them, I just never learned digital roots, so I was confused and worried I had missed something important along the way! Sigh. Up to this point I have been able to add in a LOT of conceptual learning- not a difficult stretch with CLE's good examples and all the manipulatives we own. I never let my kids do a lesson without me teaching the concept. But, I only taught first grade and we are getting into some hard math. I think I'm going to add Singapore, even if I only use the HIG. Quote Link to comment Share on other sites More sharing options...
boscopup Posted November 29, 2011 Share Posted November 29, 2011 I think I'm going to add Singapore, even if I only use the HIG. The 4A HIG (Standards Edition) has a really good explanation of divisibility rules using digit sums, including "why it works". I'm a mathy person, and I learned something new! :D Quote Link to comment Share on other sites More sharing options...
NotSoObvious Posted November 29, 2011 Author Share Posted November 29, 2011 The 4A HIG (Standards Edition) has a really good explanation of divisibility rules using digit sums, including "why it works". I'm a mathy person, and I learned something new! :D Awesome. Thanks. I've really liked the examples I've seen of the HIG. I actually was not that impressed with the workbooks (meaning, I don't think there is anything spectacular to the way the workbooks are put together- it's just practice). I do think it would be a good tool to use to balance CLE though, or vise versa. I like the review they get with CLE. My kids need that. Quote Link to comment Share on other sites More sharing options...
LNC Posted November 29, 2011 Share Posted November 29, 2011 So, apparently the way CLE teaches it is not entirely accurate! According to my super computing friend, the examples given are NOT digit sums, but digital roots, which, he says, are not very useful past divisibility tests. Interesting! It was easy enough to teach them, I just never learned digital roots, so I was confused and worried I had missed something important along the way! Sigh. Up to this point I have been able to add in a LOT of conceptual learning- not a difficult stretch with CLE's good examples and all the manipulatives we own. I never let my kids do a lesson without me teaching the concept. But, I only taught first grade and we are getting into some hard math. I think I'm going to add Singapore, even if I only use the HIG. I'm confused. Why do you say the way CLE teaches it isn't accurate? It goes into divisibility rules later too. Quote Link to comment Share on other sites More sharing options...
NotSoObvious Posted November 29, 2011 Author Share Posted November 29, 2011 I'm confused. Why do you say the way CLE teaches it isn't accurate? It goes into divisibility rules later too. Apparently the sum of the digits is a digit sum, but adding the digits again is a digital root, something slightly different. Really, it probably makes no difference for the kids, but my friend pointed out that it is technically wrong. Example: The digit sum of 98 = 17. The digital root of 98 = 8. (1+7) CLE calls both of the above a digit sum. The later is actually a digital root, something different. They just basically call two different things the same thing. Again, I don't really think it matters for the kids, but it totally explains why I was confused. Does that make sense? That was his explanation, anyway. Quote Link to comment Share on other sites More sharing options...
NotSoObvious Posted November 29, 2011 Author Share Posted November 29, 2011 When I asked on FB about the usefulness of all of digital roots, this was the answer I received from my high school math teacher: "They are useful in checking multiplication but so is grabbing a calculator." :lol: Quote Link to comment Share on other sites More sharing options...
morosophe Posted November 29, 2011 Share Posted November 29, 2011 I had no idea what a "digital sum" or a "digital root" were. Then you explained, and lo! I did indeed learn this concept as part of "casting out nines." I found it very useful. (Particularly since I was always losing my calculator. And if you have a calculator, why are you working out the problem in the first place?) They'd better move onto the application here pretty quickly, though. I can't imagine teaching digital roots without explaining all the work you'll be saving in the future when it comes to checking your work! Edited to say: Note that you can check addition, subtraction, multiplication, AND division by casting out nines. Just as long as you're working with integers, you're peachy. In fact, my first-grade son learned casting out nines for addition and subtraction this year. Quote Link to comment Share on other sites More sharing options...
NotSoObvious Posted November 29, 2011 Author Share Posted November 29, 2011 OK, now I have no idea what casting out nines is! Ha! Quote Link to comment Share on other sites More sharing options...
NotSoObvious Posted November 29, 2011 Author Share Posted November 29, 2011 Duh! I see the link! Quote Link to comment Share on other sites More sharing options...
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