Kelly's story is truly inspirational. Kelly has shown that pure determination, dedication and hard work can and does change a person's life. At Life Leisure we have loved being part of Kelly's journey.
In calculus (a branch of mathematics), a differentiable function of one real variable is a function whose derivative exists at each point in its domain.
Calculus is the study of how things change. It provides a framework for modeling systems in which there is change, and a way to deduce the predictions of such models. I have been around for a while, and know how things change, more or less.