Hello, the following fraction is the derivative of a function: (160-40t^2) / (t^2+4)^2 According to my textbook they have established the inflection points by equating the top line of this derived fraction to zero and then solving for x e.g. 160-40t^2 = 0. (t=+-2). I was wondering is it a rule that you can simply equate the top line of a fraction format derivative to zero or am I missing something particular to this equation ? Thanks for any clarification.