parallel curve (The parallel curve of parabola is not parabola)

It's very interesting that the parallel curve of parabola is not parabola..you can draw a parabola on your paper and use pen to go on by the line; you will see the parallel of the parabola will cross itself, so that's true the parallel of the parabola is not parabola; It's same to circle and sin curve;


Here is a useful formula to calculate the parallel curve and curvature and the radius of the curvature:

Here y=f(t);x=g(t); X and Y are coordinate of the parallel curve.

See more:
http://en.wikipedia.org/wiki/Parallel_curve
http://www.jstor.org/stable/3027202?seq=5