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A cone is a geometric body consisting of a plane base bounded by a closed curve (the directrix) and every point of this curve is joined to a fixed point (the apex or vertex) lying outside the plane of the base. A pyramid is a special case of a cone with a polygonal base. If the directrix is a circle and the apex is perpendicularly above the center of the circle then the cone is a right circular cone. Then the cone has a rotational symmetry around the straight line passing through the apex (the axis of the cone). Each of the line segments between the apex and the base circle is a generatrix.

The main interest of this page is to see how right circular cones can be developed into a plane.

This is a right circular cone:

Cones and Conical frustums: a cone | matematicasVisuales

The cone developing into a plane:

Cones and Conical frustums: a cone developing | matematicasVisuales

This is a plane development of a cone:

Cones and Conical frustums: plane development of a cone | matematicasVisuales

To calculate the lateral surface area of a cone we need the slant height. The slant height is the distance from the base circle to the apex of the cone (the generatrix as a segment). There is a relation between the slant height and the height of a cone (Pythagorean theorem).

Cones and Conical frustums: base radius, height and slant height of a cone | matematicasVisuales
Cones and Conical frustums:base radius, height and slant height of a cone and the Pythagorean theorem | matematicasVisuales

We are going to calculate the lateral surface area of a cone that is the area of a circular sector. If R is the base radius, the formula for the lateral surface area of a cone is like the formula for the area of a triangle. (The intuitive reason is like Kepler in Kepler and the area of a circle):

Cones and Conical frustums: lateral surface area of a cone | matematicasVisuales
Cones and Conical frustums: total surface ar| matematicasVisuales

Do you remember the formula for the volume of a cone?

A cone with its apex cut off by a plane is called a truncated cone. If this truncation plane is parallel to the base then the body is called a conical frustum.

For example, this is a conical frustum:

Cones and Conical frustums: a conical frustum | matematicasVisuales

A conical frustum developing into a plane:

Cones and Conical frustums: a conical frustum developing | matematicasVisuales

And this is its plane development:

Cones and Conical frustums: plane development of a conical frustum | matematicasVisuales

As before, we need the slant height to calculate the lateral surface area of a frustum:

Cones and Conical frustums: height and slant height of a conical frustum | matematicasVisuales
Cones and Conical frustums: height and slant height of a conical frustum (Pythagorean theorem) | matematicasVisuales

We can think, intuitively, that a cylindrical frustum is like a pyramidal frustum "with an infinite number of lateral faces". This is a very imprecise way of thinking that can remind us the origins of the Calculus, like Kepler's era. We can remind that the formula for the lateral surface area of a pyramidal frustum is like the area of a trapezoid (lateral faces are congruent trapezoids). When we calculate the lateral surface area of a conical frustum, the formula reminds us the formula for the trapezoid again:

Cones and Conical frustums: Lateral Surface Area of a conical frustum | matematicasVisuales

Cones and Conical frustums: Total Area of a conical frustum | matematicasVisuales

MORE LINKS

Plane developments of geometric bodies (5): Pyramid and pyramidal frustrum
Plane net of pyramids and pyramidal frustrum. How to calculate the lateral surface area.
Plane developments of geometric bodies (4): Cylinders cut by an oblique plane
We study different cylinders cut by an oblique plane. The section that we get is an ellipse.
Plane developments of geometric bodies (3): Cylinders
We study different cylinders and we can see how they develop into a plane. Then we explain how to calculate the lateral surface area.
Plane developments of geometric bodies (2): Prisms cut by an oblique plane
Plane nets of prisms with a regular base with different side number cut by an oblique plane.
Plane developments of geometric bodies (1): Nets of prisms
We study different prisms and we can see how they develop into a plane net. Then we explain how to calculate the lateral surface area.
Plane developments of geometric bodies: Dodecahedron
The first drawing of a plane net of a regular dodecahedron was published by Dürer in his book 'Underweysung der Messung' ('Four Books of Measurement'), published in 1525 .
Cavalieri: The volume of a sphere
Using Cavalieri's Principle we can calculate the volume of a sphere.
The volume of the tetrahedron
The volume of a tetrahedron is one third of the prism that contains it.
Volume of an octahedron
The volume of an octahedron is four times the volume of a tetrahedron. It is easy to calculate and then we can get the volume of a tetrahedron.
Chamfered Cube
You can chamfer a cube and then you get a polyhedron similar (but not equal) to a truncated octahedron. You can get also a rhombic dodecahedron.
Resources: How to build polyhedra using paper and rubber bands
A very simple technique to build complex and colorful polyhedra.