look like this-- let me do it in a different color-- the circle would look something like-- make sure it's a circle-- well, it's supposed to be centered at 0,0, but that's close enough-- so its center will be right here, and the radius if you go from the center to any point along that circle will have a distance of r, so if you go from there to there it's r, from there to there it's r, from there to there it's r, and to some degree, this formula, all it is is an extension of the distance formula, which is really just the extension of the Pythagorean Theorem.