Roulette (curve) in this case, the point lies on the circle then the roulette is a cycloid. A related concept is a glissette, the curve described by a point attached to a given curve as it slides along two (or more) given curves. Formally speaking, the curves must be differentiable curves in the Euclidean plane. The "fixed curve" is kept invariant; the "rolling curve" is subjected to a continuous congruence transformation such that at all times the curves are tangent at a point of contact that moves with the same speed when taken along either curve (another way to express this constraint
Power of a point "r" and "r" are their radii. The power of a point arises in the special case that one of the radii is zero. If the two circles are orthogonal, the Darboux product vanishes. If the two circles intersect, then their Darboux product is where "φ" is the angle of intersection. Laguerre defined the power of a point "P" with respect to an algebraic curve of degree "n" to be the product of the distances from the point to the intersections of a circle through the point with the curve, divided by the "n"th power of the diameter "d". Laguerre showed