How Tall is My Cone?

I guess I should start with the cover photo for this blog.  I have taken a Dan Meyer (look him up if you don’t know who he is…it’s definitely worth it) idea and have started using application problems as a way to get my students involved in doing math.  Not doing problems, but doing math.  I found a good trig problem involving a pyramid ruin in Egypt.  The top was missing and the students were given the angle of elevation for the pyramid and its distance from the center to the edge (including a picture with the right triangle already drawn).  That problem was the basis for this:


I started with a question and a teaser:

7.1 How Tall is My Cone_1

I had the students guess how tall the cone is and I also had the students guess how I was going to have them solve it. I’ve found that guessing is the cheapest and easiest way to get a student to buy into a lesson.

Then came the real problem:

7.1 How Tall is My Cone_2

7.1 How Tall is My Cone_3

I had and still have the cut base of the cone for them as a visual. Next came the student asking for information. I will not give them any information about the problem without them asking for it.

7.1 How Tall is My Cone_4

Then the students had a chance to work with the information they asked for. They worked in pairs and we came back together as a class and started to try to figure out what to do:

7.1 How Tall is My Cone_5

This was the work of one of the students. And this is when the bell rang. Their homework was to finish the problem. The students got an answer around 20 inches. I then showed them the answer:

7.1 How Tall is My Cone_6

Then we had a discussion about why thier answer wasn’t correct. Someone finally made a point about they didn’t add the base in (but that only made their prediction worse). After some deliberation, someone relaized that we calculated the height as if the top of the cone came to a point. Turns out, it doesn’t. Someone came to the board and extended the cone with two line segments and decided that’s why they were a couple of inches off.

There could be an interesting follow-up when we get to solids!


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