Alice Posted February 2, 2012 Share Posted February 2, 2012 I need to explain to my son why A -B + C = (A-B) + C but not A - (B + C) He is only 8 and only in Singapore 4A. So I need to explain it fairly simply. The reason it came up is that he did a problem wrong and I realized that the reason why is he did it the second way (added B and C together and then subtracted from A). I can just give him the rule that you have to do all subtraction and addition left to right. But I'm trying to understand better myself WHY so that I can explain it better to him. This is the kind of thing that was left out of my own math education. I figured I could also get a bunch of manipulatives out and we could play with them to show him visually. Still, I thought maybe there would be some math guru on here that could explain the why behind it to me. :) Quote Link to comment Share on other sites More sharing options...
Farrar Posted February 2, 2012 Share Posted February 2, 2012 This is order of operations and I actually asked much the same question here a month or so ago and got a good set of resources and a nice answer. Here's that thread if you're curious. I felt like I could follow order of operations but I didn't get the why behind it (and was therefore always forgetting it). While I got a nice answer there from Dana that helped me get it - I also learned that to some extent, it's just the convention - math people have just decided that's the order. Quote Link to comment Share on other sites More sharing options...
Alice Posted February 2, 2012 Author Share Posted February 2, 2012 Oh, thank you! You know I should have thought to do a search here. I did search for order of operations on google and found a few sites but they were either of the here's the mnemonic variety or really really complicated math that was just not going to help me explain it to my son. That thread was very helpful, so thanks! Quote Link to comment Share on other sites More sharing options...
NittanyJen Posted February 2, 2012 Share Posted February 2, 2012 I need to explain to my son why A -B + C = (A-B) + C but not A - (B + C) He is only 8 and only in Singapore 4A. So I need to explain it fairly simply. The reason it came up is that he did a problem wrong and I realized that the reason why is he did it the second way (added B and C together and then subtracted from A). :) Simple answer: A - (B + C): You are saying with parenthesis that you want to add B and C first, then subtract that sum from A. If you do that, you have to subtract everything inside the parenthesis, yes? When grouping (A-B) + C, you are only subtracting B from A, because again, you do what is inside the parenthsis first. In the original problem, A - B + C, the only thing being subtracted is B. C is being added as you work from left to right, one item at a time. You can do this with manipulatives, or on a number line. Hope that helps :) Jen Quote Link to comment Share on other sites More sharing options...
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