why the formula for circumference is expressed as 2pi r rather than simply pi d? Anyone know?
Well, it sometimes is expressed as πD.
I'm just guessing here, but I think the radius is considered a more fundamental parameter for a circle than the diameter. The circle is the set of points that is all the same distance from a central point, and that distance is the radius. (There are, incidentally, many shapes other than the circle that have a constant diameter, but only the circle has a constant radius...).
Of course, that just changes the question: why was the symbol π defined as referring to the ratio of circumference to diameter, rather than the ratio of circumference to radius, 6.28... It really should have been; when you think of it, it is really the value 2π that keeps cropping up everywhere. Dpbsmith 15:28, 24 April 2007 (EDT)