Given a point d where the function f is continuous,
let's calculate the incremental ratio of the function . We use some properties of the Riemann integral
and the mean value theorem.
where μ is the mean value of the function f between x and d. This value is enclosed between the infimum and the supremum of the function in the interval [x,d] or [d,x]. The continuity of the function implies that μ tends to f(d) as x tends towards d. This proves the theorem.