1. The problem statement, all variables and given/known data Let α(t) be a regular, parametrized curve in the xy plane viewed as a subset of ℝ^3. Let p be a fixed point not on the curve. Let u be a fixed vector. Let θ(t) be the angle that α(t)-p makes with the direction u. Prove that: θ'(t)=||α'(t) X (α(t)-p)||/(||(α(t)-p)||)^2 2. Relevant equations 3. The attempt at a solution I'm not really sure how to approach this problem. I know what it is asking though. I have tried to extend the tangent line to the point of intersection and meeting it with u to make a triangle and applying the law of cosines but that didn't get me anywhere. I suspect this problem will ask me to use the angle definition of cross product: aXb=absin(θ) but I really don't know.