The fundamental theorem of calculus (part 1)
If $f$ is continuous and $F(x) = \int_0^x f(u) du$ then $F'(x) = f(x)$.
The top figure shows the graph of $f(x)$ with the pink area representing the definite integral from $0$ to $x$, or $F(x)$, as defined by the definite integral. The red area is $F(x+h) - F(x)$.
The graph of $F(x)$ appears in the middle figure. As $f(x) > 0$ this function is increasing,
The bottom figure shows the graph of $f(x)$ using a dotted line and the graph of $(F(x+h) - F(x))/h$. As can be seen, this approximate derivative has the same graph of $f(x)$, the differences being masked by the pixel size.
$c =$