I have some data in a text file. I want to import this data into Mathematica, and then I want to calculate the numerical derivatives of this data. In particular, I need to find the y value where the first derivative is equal to zero. I can import data. I can use the ND function for numerical derivativess, but I'm unsure how to accomplish the rest. The data follows a sine curve with increasing amplitude. Do I need to fit the data first somehow? Any help is appreciated. I'm kind of a beginner when it comes to Mathematica.