1. The problem statement, all variables and given/known data Let alpha(t) be a regular curve. Suppose there is a point a in R^3 space s.t. alpha(t)-a is orthogonal to T(t) for all t. Prove that alpha(t) lies on a sphere. 2. Relevant equations Definations: A regular curve in R^3 is a function alpha: (a,b)-->R^3 which is of class C^k for some k (greater than or equal to) 1 and for which the time derviative does not equal zero for all t in (a,b) Note: For this book all classes will be C^3, unless stated. (it is not stated otherwise in this problem so assume C^3). T(t)= Tangent vector field to a regular curve as defined by (d(alpha)/dt)/norm(d(alpha)/dt)). Hints: what should be the center of the sphere? 3. The attempt at a solution And here is where I get stuck, I have not been given a crietria for a curve to lie on a sphere (and if I had oh so many years ago I have long since forgotten it). All of my attempts so far have fallen apart due to assumptions I have had to make, which without a guide towards showing something lies on a curve, I am not sure are justified. Just a hint, aside from the one given, is all I need to get some of my juices following. I know it isn't proper form to not show my solution; however, it won't be of use as I don't really know where I am heading with it. Any help would be great.