3 ms·

Why isn't this type of mental calculation actually taught at schools? Not every kid thinks of these "tricks". Instead, they do how the teacher showed them and d

by dsego 13y ago

Why isn't this type of mental calculation actually taught at schools? Not every kid thinks of these "tricks". Instead, they do how the teacher showed them and do it on paper the long way. Seems like we are crippling our kids.

raverbashing 13y ago
Because the teacher a) can't b) don't care and c) shuns those who point their ignorance Yes, the way of speeding these calculations is to break it in easy steps (which might involve some simple calculations themselves)
Gmo 13y ago
Well, I was educated in France and I was taught those tricks.
adestefan 13y ago
That's how I was taught in the mid-80s at a Catholic elementary school in Pennsylvania. I believe I heard, probably on NPR, that the new Common Core standard also recommends teaching in this manner.
- neltnerb 13y ago
  I heard as well that teachers are moving to a more sane method to teach multiplication. The best part about MSB math, to me, is that at least you get a close approximation to the answer quickly. It's like successive approximations, the errors from making a mistake go down as you move along if you don't need a precise result. For instance, what's pi * e? Good luck calculating that from the LSB... ;-) But with MSB, you know it's about 9 because pi is on the low end of 3-4 and e is on the high end of 2-3. So you can do 3.14 * 2.72 33 + .143 - .28*3 - something small (idea here is that the cross term is small, three sig figs) 9 + .4ish - .9ish 8.5ish Actual result is 8.5397. So you can get a few sig figs by saying "eh, these digits are too small to matter" and ignoring cross multiplication of the LSBs for problems that aren't for the purpose of arithmetic. I use this often in engineering estimation on the fly, because so many constants are only known to 1 sig-fig anyway that it's really hardly worth worrying about more accuracy. Particularly when you're building in a safety factor anyway.

by dsego 13y ago

raverbashing 13y ago

Because the teacher a) can't b) don't care and c) shuns those who point their ignorance Yes, the way of speeding these calculations is to break it in easy steps (which might involve some simple calculations themselves)

Gmo 13y ago

Well, I was educated in France and I was taught those tricks.

adestefan 13y ago

That's how I was taught in the mid-80s at a Catholic elementary school in Pennsylvania. I believe I heard, probably on NPR, that the new Common Core standard also recommends teaching in this manner.

neltnerb 13y ago
I heard as well that teachers are moving to a more sane method to teach multiplication. The best part about MSB math, to me, is that at least you get a close approximation to the answer quickly. It's like successive approximations, the errors from making a mistake go down as you move along if you don't need a precise result. For instance, what's pi * e? Good luck calculating that from the LSB... ;-) But with MSB, you know it's about 9 because pi is on the low end of 3-4 and e is on the high end of 2-3. So you can do 3.14 * 2.72 33 + .143 - .28*3 - something small (idea here is that the cross term is small, three sig figs) 9 + .4ish - .9ish 8.5ish Actual result is 8.5397. So you can get a few sig figs by saying "eh, these digits are too small to matter" and ignoring cross multiplication of the LSBs for problems that aren't for the purpose of arithmetic. I use this often in engineering estimation on the fly, because so many constants are only known to 1 sig-fig anyway that it's really hardly worth worrying about more accuracy. Particularly when you're building in a safety factor anyway.