How many edges? Hmm, I’m not sure. Aren’t edges supposed to it is in straight?

How countless faces? That’s easy! One. There’s a circular confront on the bottom. However that’s no a polygon, therefore is that still a face? Oh, and also what perform I contact the various other surface top top the cone? Don’t faces have to be flat?

A common question we obtain from teachers in qualities 1, 2, and also 3 needs to do with exactly how to describe the characteristics of details three-dimensional solids, specifically cylinders and cones. According to the TEKS, college student are supposed to describe three-dimensional solids making use of formal geometric language such as vertex, edge, and face. The problem is that we’re trying to use language that functions for one course of shapes to define the features of a fully different class.

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Three-dimensional shapes like prisms and also pyramids room polyhedrons. “In geometry a polyhedron is merely a three-dimensional hard which consists of a repertoire of polygons, typically joined at their edges.” (Source) these solids have “flat polygonal faces, right edges, and also share corners or vertices.” (Source)

Spheres, cylinders, and also cones, top top the other hand, are not polyhedrons. Together a result, we can’t usage the precise same language to explain them, or if we execute use the very same language it’s v the expertise that the definitions are not identical. Take the word vertex, for example.

On a rectangle-shaped prism, a vertex is the sharp point or corner where edges meet. A rectangular prism has actually 8 vertices.

However, this very same term can additionally be offered to explain the allude of a cone. Exact same term, however not the exact same definition. Together Dr. Mathematics says,

The really tricky component here is the the “vertex” of a cone has actually nothing to perform with edges, for this reason it requirements a whole brand-new definition; and I can’t think the a really good elementary-level definition for what they obviously mean, i beg your pardon is simply a “point.”

As college student pursue an ext advanced mathematics, they have the right to develop an ext sophisticated language and definitions. In the meantime, while they room in primary school school, we use the term vertex that a cone in RRISD to describe this attribute of a cone.

If we want students come describe and classify this type of three-dimensional solid, climate we have to provide easily accessible language for that purpose.

What about the other attributes of a cone? Again, our score is to administer language the is accessible because that elementary students and also descriptive of this attributes, recognizing that our students will develop much more formal understandings later on in their institution careers. To explain a cone, we would say that it has a circular base, the flat surface upon i m sorry the cone rests. We also say that it has actually a curved edge along the base and also a curved surface the extends native this edge approximately the vertex.

What around a cylinder? currently that we have available language come describe characteristics of a cone, we can extend this language to describe attributes of cylinders.

The above cylinder is created of two circular bases, one ~ above top and also one ~ above the bottom. It likewise has 2 curved edges, one along the top and one follow me the bottom. Finally, it has actually a curved surface that extends from the bottom edge up to the top edge.

I should add that both the cone and also cylinder I’ve explained are a appropriate circular cone and also right cylinder. Just like polygons and also polyhedrons, there are plenty of other kinds of instances of this shapes. For example, the cone or cylinder can slant, making castle oblique.

Source: plane and solid Geometry, George Wentworth, 1899, pages 332 and 342

It is essential for college student to check out a selection of instances of two- and also three-dimensional figures. The more they encounter, the much more they have to face their definitions and terminology which serves to combine their understanding of the attributes and how they help us identify and classify this figures.

So what go this look favor on STAAR?

On the 2016 released test, STAAR request a question that handle this very topic and also reinforced the vocabulary we are using in RRISD.

The correct answer is F They have no vertices. If you look at collection B, you’ll notification that it has a cone, which together we discussed earlier does have a vertex. If the Texas Education company were not using the hatchet vertex the a cone, climate we likely would have actually seen the cone consisted of in collection A.

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Here’s a parting assumed from Dr. Math:

What definition you use depends on what you are going to execute with it. If you are simply describing objects, my loose meaning is fine. If you room going to prove theorems involving planes and angles, you’ll want to restrict you yourself to the polygonal definition, but then you won’t it is in asking any type of questions around cones. I think human being often failure to realize that also though we are particular about meanings in math, those definitions vary from ar to field, as they are adjusted to a specific context. That’s what I’m trying to execute here.