My oldest ( 8yrs ) is curerntly in 8th grade has been working furiously with his white board doing math and has been doing algebraic equations
Today, he just gets out of bed ( 6:30 am ) and says there is another way to do multi-digit multiplication. he says it works best when the numbers are in the same 10s group [ ie 14 x 19, or 145 x 142 or 1567 x 1562 )
He gives this example :
211 x 215
You take the one's place digit.
211 = 210 + 1
215 = 210 + 5
210 = z
1 = a
5 = b
211 x 215 = ( z+a) (z+b)
(z+a)(z+b) = z^2 + zb + za + ab
z^2 + zb + za + ab = z(z+b) + a(z+b)
I told him that could be done with 2 digit problems as well, he said it was just quicker to do it the regular way than this way
So when faced with a multi-diigit multiplication problem with numbers in the same 10s group just use z(z+b) + a(z+b)
I smiled at him and said good job, and I sent him off to figure out how to make this equation work for multi-digit division ...
This is not the first time he has done this ....
Before he showed me this a couple weeks ago.
( b√x )^(b-1) = x ÷ b√x
I asked him where in the book that he was reading that was stated ... he said it isn't .. he just looked at the numbers and figured it out. I read the book he was reading .. it was not stated anywhere.
So we did 3 examples to prove this and it does work
So now it seems I need to start reading some math books ... and refresh my knowledge from that math I took in high school and college.
Last edited by Cawdor; 12/27/12 09:38 AM.
