If n is a positive integer, the power rule says that the derivative of x^n is nx^(n-1) for all x, whether you are thinking of derivatives at a point (numbers) or derivatives on an interval (functions). This can be derived using the binomial theorem or product rule. Similarly, if n is zero or a negative integer, the power rule says that the derivative of x^n is nx^(n-1) for all nonzero x.