What is the first derivative used for?

Answer 1

The first derivative f'(x) gives the instantaneous slope of f(x). If f'(x) is positive, then f(x) is increasing (positive slope), and if f'(x) is negative, then f(x) is decreasing (negative slope). If f'(x) = 0, then the graph of f(x) is flat at the point (slope = 0).