Try using the geometric definition of the accumulation function to determine its derivative. Start by sketching a typical positive, continuous function $f$. Let $F$ denote the accumulation function, which we can interpret as the area under the graph of $f$ above an interval $[a, x]$.
a. Because we intend to establish a basic derivative formula for a new function $F$, we will need to go back to the definition of $F^{\prime}(x)$ as the limit of a difference quotient. Write the formula for $F^{\prime}(x)$ using the definition of the derivative.
b. Interpret the numerator as the difference between the areas of two overlapping regions. Sketch an example and shade in the vertical strip whose area corresponds to the numerator. Interpret the denominator as the width of the vertical strip. What is the geometric significance of the quotient?
c. How does the limit of this quotient compare with the value of $f$ at $x$ ? State your conclusion as a general formula for $F^{\prime}(x)$. Congratulations! You have just given an intuitive proof of the Fundamental Theorem of Calculus, which some people think is the most important intellectual achievement of recorded history.