No-carry subtraction
February 14, 2008 am29 12:15 am
Does anyone else do this:
3418 – 1279 = 3418 – 1300 + 21 ?
(2118 + 21 = 2139)
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JD2718 
My son did a science project about 7 or 8 years ago about the abacus, and I believe that was a key technique in learning subtraction.
I forgot, mathmom’s son does this…
That’s how I tend to do subtractions mentally, at least those that lend themselves to it.
Probably do it almost every day. Never write it out, but anytime I’m doing mental math.
Yep – I do that too. Mental subtraction seems harder than addition (to me at least)
I do 3418 + 21 – 1300.
I actually think of it more as adding the same amount to both numbers,
x-y = (x+n)-(y+n),
choose n to make y+n as nice as you please.
Sometimes I do something even weirder, maybe like 3418-1279 = 3319 + 99 – 1200 – 79 = (3319 – 1200) + (99 – 79).
Breaking off 99s makes the subtraction easier sometimes.
The problem with our calculator generation is that kids don’t think of doing things like that and they are not shown these methods.
My AP calculus kids are soemtimes in awe of some of the arithmetic tricks I show them.
I do it routinely for mental math, and its one of the first tacks I take when my daughter tries to use me has a human calculator… Actually I’m more iterative with that… With her I’d probably do “What’s 3418-1200? What’s 3418-1270? etc”
Y’all should go listen to Tom Lehrer’s “New Math” recorded back in ’63 or ’64.
http://www.youtube.com/watch?v=tx5KDyvlG3Q&feature=related
The person on the vid is obviously NOT Tom Lehrer, but someone lip-synching the song. Lehrer was a Harvard mathamatics professor who happened to be a very good musical satirist. Enjoy.
Somewhere, somewhere, I have a warped 33… Not that album… But mine has an even more mathematical song: “…Metro Goldwyn Moskva buys movie rights for 6 million rubles, changing title to ‘The Eternal Triangle’ with Ingrid Bergman playing part of the hypotenuse…” I talk about this one in class…
By the way, that is Lehrer’s voice singing on the video.