Any idea what this shape is called?

It came up in a lesson before Christmas and I didn’t know what it was called. The student who first drew it didn’t want it named after him so as it was my housemates birthday I called it Bob and promised to find out its real name. Turns out after quite a bit of discussion and googling no one in the maths staff room could come up with a better name. I really like the idea that there are still things in mathematics that might not be named yet.

The students in my booster class got quite into the idea of trying to give it a more mathematical name. A couple of the tables started calling it a cone as they saw it as being the net of a cone. I was discussing this with them for a bit and they were really convinced that’s what it should be called but I wasn’t so we tried cutting some out. Bob doesn’t make a cone though and neither does their other suggestion. Their new name for it was “the flattened not cone”. Somehow I don’t think that’s going to catch on.

This activity has been one of my favourite starters because of all the discussion and genuine interest it generated. Here are some of the responses that the students stuck up on the boards at the end.

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January 10, 2015 at 11:12 pm

I found this quite interesting when trying to think of how to search for the shape name. It made me think about the properties of the shape and look at the definitions of the parts of a circle and of triangles more closely. I don’t have a name for you, but I would describe it as a shape combining a segment and a triangle (and of course in this case, an isosceles triangle).

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