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Archive for August 2011

I know I’m doing it right when…

by Max
August 10th, 2011

1) Students ask mathematical questions (that in turn drive their learning)
2) Students and teachers seem happy to be in class and proud of their contributions
3) Students and teachers relate what they learn to what they already know (in math and in their lived experience) and to what they want to know more about/do better (in math and in their lived experience)
4) Students and teachers collaborate to learn and make sure everyone is learning
5) Students and teachers communicate their ideas fluently out loud and in writing
6) Students solve novel problems with a variety of strategies, tools, and representations
7) Students and teachers ask not just “am I right?” but “how do I know?”
8) Students and teachers ask, “what other math/patterns/generalizations can I discover from this?”
9) Students and teachers ask, “what are we learning about? what’s the big idea? what do we need to practice to get better?”

What’s missing? What’s superfluous/not fundamental? How do you know that your classroom looks like you want it to? How does this relate to what you assess?

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I Get Helped!

by Max
August 5th, 2011

I was trying to solve this problem and stumbled on a subproblem. What is the probability that one person does NOT see their name when it is in one of 100 identical boxes and they can open 50 of them?

My gut told me there is a 50% chance I don’t see my name, but another part of me said, there is a 99/100 chance I don’t see my name on the first try, a 98/99 chance I don’t see it on the second try, etc. Should I add or multiply all those numbers? How?

So, I turned to Twitter. Alexander Bogomolny put an explanation on his Facebook page that convinced me my initial 50% hunch was right and helped me see that I wasn’t accounting for the probability that I have to even try a 2nd, 3rd, 4th, etc. time.

Shawn Urban helped me realize why I wanted to be adding, not multiplying, all those probabilities (when adjusted with Alexander’s suggestion).

Earl Samuelson showed another way to confirm that 50% is the correct answer that fit into my most basic understanding of probability: # of favorable outcomes / # of possible outcomes. The total number of possible outcomes is all the ways to choose 50 boxes out of 100 (100 C 50 on a graphing calculator). The total number of successful outcomes are all the outcomes where you pick the 1 box with your name in it from its 1 location (1 C 1) and 49 other wrong boxes out of the remaining 99 (99 C 49). So the probability of picking your name is (1 C 1 * 99 C 49) / (100 C 50) = .5, and the probability of not picking your name is 1 – .5 = .5

I really, really love it when multiple solutions and ways of reasoning through a problem yield the same result. I often learn something about deeper patterns in math when that happens.

Thanks Twitter!

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I’m Stumped!

by Max
August 5th, 2011

Working on this problem:

100 people are in a room. In the next room, there are 100 identical boxes. Each box contains one person’s name (and each person’s name is in exactly one box). One at a time, each person can go into the box room and open up to 50 boxes. Then, they must return the box room to its original state (no re-ordering boxes, no marking boxes in any way). They leave the room and don’t communicate with anyone else. Individually, each person is asked, “which box had your name.” Fabulous riches are showered down on the group if all 100 people answer correctly. Are there strategies they can use to improve the odds that they answer right?

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