athomeontheprairie Posted October 23, 2013 Share Posted October 23, 2013 We are on lesson 40, and they are introducing "check facts". This is ridiculous to me. Is it necessary? Does it build on this concept later? They very first problem my dd attempted was 15-7=8 which according to Rs was 6-7=8 (our maybe 8-7=6). Either way wrong answer. Have you used RS D? What did you do? Should I proceed and us this a necessary skill? Or skip it a complete rubbish? Quote Link to comment Share on other sites More sharing options...
Goldilocks Posted October 23, 2013 Share Posted October 23, 2013 The check numbers are pretty neat. Much more is done with it at the end of D and throughout E, but it is only a suggested way to check your answers. Not the actual math lesson. For subtraction you need to add. So, for 15 - 7 = 8 The check numbers become (6) (7) (8). Turn that around to 8 + 7 = 15. The check number for 15 is 6. So it works out. Obviously this is pointless with these numbers. But when you are doing multi-digit multiplication, the check numbers can be useful. Quote Link to comment Share on other sites More sharing options...
neeko Posted October 23, 2013 Share Posted October 23, 2013 Timely - we are just starting with RS-E and hit check numbers for the first time yesterday. My poor dd's head nearly exploded. We played the check number game (A63?) today before trying them with subtraction and things got a little better. She's starting to see the point, but it was ugly for awhile there. Quote Link to comment Share on other sites More sharing options...
Aloha2U Posted October 23, 2013 Share Posted October 23, 2013 Proceed. It's a useful, helpful skill. You'll be glad for it in the long run, especially if/when you get to multivides in E. Quote Link to comment Share on other sites More sharing options...
boscopup Posted October 23, 2013 Share Posted October 23, 2013 These are digit sums, and they're very useful when you get to divisibility. CLE teaches this also, and people often get stumped when they first see that lesson. :D Try it with bigger numbers, and you'll see it's pretty cool. I personally never use them to check my addition/subtraction, and I wouldn't use them for basic math facts like 15-7=8, but I use them for divisibility purposes for sure. ..23438 -> 2 - 11111 -> 5 _____ ..12327 -> 6 6+5=11, whose digit sum is 2. Quote Link to comment Share on other sites More sharing options...
Soror Posted October 23, 2013 Share Posted October 23, 2013 I skipped it at first but then we went back to it later. Glad to read it will be useful in E. Quote Link to comment Share on other sites More sharing options...
ScoutTN Posted October 23, 2013 Share Posted October 23, 2013 I used to use these to check for errors when I was a bank teller. Back in the dark ages before computers did all the work! Quote Link to comment Share on other sites More sharing options...
athomeontheprairie Posted October 23, 2013 Author Share Posted October 23, 2013 Thank you for all your answers! I suppose I should learn the skill so that I can teach it to her... Quote Link to comment Share on other sites More sharing options...
Korrale Posted October 23, 2013 Share Posted October 23, 2013 Maybe this will help. http://youtu.be/qva8BG13Klc Here is more information. en.wikipedia.org/wiki/Casting_out_nines Quote Link to comment Share on other sites More sharing options...
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