I was dazed when an interviewer asked this question at an interview I attended some days back. I did flop, I knew. So, I posted this discussion on a Nigerian engineers forum and was dazed by some mind blowing responses I got. In two sentences, you tell why Engineers study dy/dx. You may want to see answers to this question proffered by practicing engineers http://go.engineer-ng.net/forum/topics/why-do-nigeria-engineers-study-dy-dx-nigeria-engineering?xg_source=activity. Drop your response on this thread and maybe there.