If you look closely at our original derivative rules, we see that they were designed to differentiate functions; that it equations whose graphs pass the vertical line test. However, even equations whose graphs do not pass the vertical line test (such as a circle) clearly have slopes at particular points. So how do we find the slope of a curve that does not represent a function? Ans: Implicit Differentiation. In this video we will look at the process of implicit differentiation and how to take the derivative of an equation that does not represent a function.