1. The problem statement, all variables and given/known data Show that if σ(t) for (t in I) is a parametrization of a line, then σ''(t) is parallel to σ'(t). 2. Relevant equations 3. The attempt at a solution I thought that if σ(t) is a parametrization of a line then it could be expressed as σ(t) = vt + a, but then σ'(t) = v and σ''(t) = 0. Can two vectors be parallel? Is it because the distance between them is constant? Or did I make a mistake earlier on?