Volume of Right Circular Cone

Theorem
The volume $V$ of a right circular cone is given by:
 * $V = \dfrac 1 3 \pi r^2 h$

where:
 * $r$ is the radius of the base
 * $h$ is the height of the cone, that is, the distance between the apex and the center of the base.

Construction
Construct the following triangle:


 * ConeVolumeProof.png

Let $A$ be located at the origin of the $xy$-plane.

By definition of tangent, the line segment $AB$ can be described by the equation $y = \dfrac {BC} {AC} x$.

By Euclid's definition of a cone, the solid of revolution generated by rotating $\triangle ABC$ about the $x$-axis is a right circular cone:
 * whose axis is $AC$
 * whose base consists of the circle whose center is $C$, whose radius is $BC$ and whose plane is perpendicular to $AC$.

As $AC$ is perpendicular to the base of the cone, the height of the cone is $AC$.

Let $h = AC$ denote the height and $r = BC$ denote the radius of the base.

Proof
This proof utilizes the Method of Disks and thus is dependent on Volume of a Cylinder.

From the Method of Disks, the volume of the cone can be found by the definite integral:


 * $\displaystyle (1): \quad V = \pi \int_0^{AC} \left({R \left({x}\right)}\right)^2 \ \mathrm d x$

where $R \left({x}\right)$ is the function describing the line which is to be rotated about the $x$-axis in order to create the required solid of revolution.

In this example, $R \left({x}\right)$ describes the line segment $AB$, and so:


 * $R \left({x}\right) = \dfrac r h x$

We have also defined:
 * $AC$ as the axis of the cone, whose length is $h$
 * $A$ as the origin.

So the equation $(1)$ is interpreted as: