Thursday, September 22, 2016

How my students discovered the distance formula

My focus in geometry has been on conceptual understanding, so in introducing distance formula, I wanted to make sure students understood the why. It took me two days, but my students have really grasped this concept, so I am very proud of them.

I started with a review of Pythagorean Theorem. We reviewed the puzzle and did some practice with this Tarsia puzzle {a freebie in my TpT store!}

My students did these pennants from Scaffolded Math and Science, which have awesome questions to get kids thinking about Pythagorean Theorem (and they look great as classroom decor). I love the questions that start my giving students the area of the right triangle a side length and ask them to work backwards to the side lengths. I also love that this student made Pythagoras a red head - like me :)


Once we had the mechanics of the Pythagorean Theorem ironed out, we put it onto a coordinate plane. We graphed two points and I asked students if they could see a way to apply the Pythagorean Theorem. We had a great discussion about how it didn't matter if you drew the right triangle above or below the segment. One student even pointed out, "You are always going to be missing the hypotenuse because you can't count the diagonal distance." (Which I was SO excited to hear a student articulate. I can't tell you how many times I have seen students try to count diagonally across the graph.) We did two examples in their INBs and then I gave them this Distance Formula Maze. I printed them one 1/4 of a page and attached graphs to the sheet. I told them to complete the first five problems using Pythagorean Theorem and then stop.


As they worked, I asked them to think about whether they could do this problem without a graph and what that would look like. I also told them I would give a Dum-Dum to any really good answers, so that got students extra motivated. One student said, "Draw a graph!" I told him that would be a great strategy on the EOC, but I wanted them to try it without graphing anything, so they went back to thinking.

One student called me over and said, "Couldn't you just find the difference between the y-values and x-values in your ordered pairs and then use those numbers to do Pythagorean Theorem?" I asked her to prove this would work with an example she already did, so she looked a problem one, where she found the distance between (4,7) and (8,-5). She said, "Well 7 - (-5) is 12, so ... oh look, that's the same as the length of the leg. And 8-4 is 4 - so I think that would work." Then another student called me over and said, "This reminds me of learning about slope in Algebra class and this formula  y2-y1 / x2-x1. Expect you wouldn't want to divide them at the end." I was so excited  - here were my students coming up with ideas and connecting to prior knowledge all on their own. We flushed out these ideas with a group discussion and came up with the distance formula. They saw the connection between Pythagorean Theorem and the Distance Formula and my textbook has this cool graphic that helps really make that connection.












Then we did some examples and I sent them back to their maze. I told them to try our new formula for the next five and then they could finish any way they wanted.

As students practiced, they started to favor one or the other. A few clever students came up with a hybrid of their own. One student was so proud he called me over and asked if it was OK for him to make up his own distance formula. I was intrigued, so he demonstrated with the points (1, -4) and (7, -1). He drew a table to help him find the distance between the x and y-values and then plugged those numbers into Pythagorean Theorem. Talk about taking ownership of your learning!

Our next lesson was midpoint, which I used my fan lesson that I blogged about here. Again, the students understood what they were doing. Today, I gave them this mini quiz with a tough Check All That Apply problem, and they nailed it! Discovery learning and conceptual understanding
for the win!

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