Day 21 Play-by-Play, MAT115

Today was all about Egyptian stuff. In particular, we did a review of the multiplication we started last time, and checked that multiplication works whether we double the bigger (which I suggest, all else being equal) or double the smaller.
So the first way we did the product 23*42,

23=16+4+2+1

1 42 *
2 84 *
4 168 *
8 336
16 672 *
32 Too big!

672+168+84+42 = 966

Build on the left Doubling in the middle Answers on the right

Gives the same answer as if we'd doubled 23, instead:

42=32+8+2

1 23
2 46 *
4 92
8 184 *
16 368
32 736 *
64 Too big!

736+184+46 = 966

Build on the left Doubling in the middle Answers on the right

We then did a little bigger problem, 321*112:

112=64+32+16

1 321
2 642
4 1284
8 2568
16 5136 *
32 10272 *
64 20544 *
128 Too big!

20544+10272+5136 = 35952

Build on the left Doubling in the middle Answers on the right

Multiplication's not too bad. How about Egyptian division? I've assigned this reading to do while working on your homework (see the daily agenda for more about division, and how it's just multiplication backwards).

Let's look at some problems. (I'll come back to these when I get my iPad back, and can reference the work we did today in class.)

Here's an annotated version of the Day 21 agenda, and
the video that corresponds to that annotation.

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