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3D model of regular octahedron. The surface area of a regular octahedron can be ascertained by summing all of its eight equilateral triangles, whereas its volume is twice the volume of a square pyramid; if the edge length is , [12] =, =. The radius of a circumscribed sphere (one that touches the octahedron at all vertices), the radius of an ...
The basic quantities describing a sphere (meaning a 2-sphere, a 2-dimensional surface inside 3-dimensional space) will be denoted by the following variables r {\displaystyle r} is the radius, C = 2 π r {\displaystyle C=2\pi r} is the circumference (the length of any one of its great circles ),
The area of a regular polygon is half its perimeter multiplied by the distance from its center to its sides, and because the sequence tends to a circle, the corresponding formula–that the area is half the circumference times the radius–namely, A = 1 / 2 × 2πr × r, holds for a circle.
Graphs of surface area, A against volume, V of the Platonic solids and a sphere, showing that the surface area decreases for rounder shapes, and the surface-area-to-volume ratio decreases with increasing volume. Their intercepts with the dashed lines show that when the volume increases 8 (2³) times, the surface area increases 4 (2²) times.
This formula can also be derived without the use of calculus. Over 2200 years ago Archimedes proved that the surface area of a spherical cap is always equal to the area of a circle whose radius equals the distance from the rim of the spherical cap to the point where the cap's axis of symmetry intersects the cap. [3]
The formula for the volume of the -ball can be derived from this by integration. Similarly the surface area element of the -sphere of radius , which generalizes the area element of the -sphere, is given by
Apollonius of Perga discovered the curious result that the ratio of volumes of these two shapes is the same as the ratio of their surface areas. [11] Both volumes have formulas involving the golden ratio, but taken to different powers. [12] As it turns out, the icosahedron occupies less of the sphere's volume (60.54%) than the dodecahedron (66. ...
Its surface area is four times the area of an equilateral triangle: = =. [7] The volume is one-third of the base times the height, the general formula for a pyramid; [7] this can also be found by dissecting a cube into a tetrahedron and four triangular pyramids. [8]