what is the equation of a disk?

Answer 1

A disk? As something like a 3D coin? Okay:

z = (±a/b) ((b/2)^c - (x² + y²)^c)^(1/c)

where a is the thickness of the disc, b is the diameter of the disc, and c is some arbitrarily large number. The higher c is, the more square the edges of this disc will be, so it would be more like a coin instead of a pill.

Answer 2

Assume the 3 major axes all perpendicular to one another, x,y,z
Let t be the uniform thickness of the disk, r the radius
Let the equation of the disk be z= z(x,y) ie z is a function of x,y
z = does not exist if sqr[x^2+y^2] > |r|
z = t if sqr[x^2+y^2] <= |r|
note that the absolute value for r does not really need to be here, I just do that to remind myself that r is always >=0