Take two numbers. Make them reasonably sized, two or three digits. Longer will work, but we're just using this as an example. I'm gonna use 42 and ... uh ... 421.
42 x 421 = ???
Now, take the number on the left and double it. The number on the right should be cut in half. If the new number on the right isn't an integer (i.e. if it ends in point 5 [.5]) then drop everything to the right of the decimal.
42 x 421 = ???
84 x 210.5 = ???
84 x 210
84 x 210 = ???
Everything clear so far? Too bad, we're pushing on. Repeat what you did above until the number on the right equals 1.
42 x 421 = ???
84 x 210 = ???
186 x 105 = ???
372 x 52 = ???
744 x 26 = ???
1488 x 13 = ???
2976 x 6 = ???
5952 x 3 = ???
11904 x 1 = ???
Now, cross out every line where the number on the right is an even number.
42 x 421 = ???
168 x 105 = ???
1344 x 13 = ???
5376 x 3 = ???
10752 x 1 = ???
Now, add all the remaining numbers on the left.
42 + 168 + 1344 + 5376 + 10752 = 17,682
Punch the original equation into a calculator and see what you get?
4 comments:
This is somehow easier? *wrinkles brow*
You don't have to know how to multiply. You just add stuff.
Yes, but you still have to add a lot of stuff!
I started blogging again, if you're interested.
ba-blog-ba.blogspot.com
I used to be an elementary education major. I took these math classes that covered 7 different ways to multiply and 7 ways to divide. They cover all these different approaches because not all kids learn to multiply the same way.
Multiplication seems simple to us now, but back as kids it was a struggle. Those Russian peasants often didn't have the education that we had and needed a trick.
Here I'd call it DC High School Student Multiplication.
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