1. The problem statement, all variables and given/known data [itex]r = 2\cos(\theta)[/itex] 2. Relevant equations 3. The attempt at a solution Hello, please do not evaluate. Why do textbook state that the derivative of the polar function (symbolic) is dy/dx and not [itex]dr/d\theta[/itex]? It is a function of theta, then why is the derivative dy/dx? Idea: Even though the equation is polar, you are defining the function in the xy-plane, so therefore, the function really is parametric y = f(x), but the parameter is theta?