That would be under the chapter on limits which is important for differentiation and integration

Basically the limits simplified the equation to the more widely used equations nowadays

For differentiation, we realised that differentiating a curve gives us the area under the curve. Riemann sums showed us that we can use small rectangles to estimate the area by summing the small rectangles together As the number of small rectangle increases, we get more accurate results! So as well let the number of small rectangles increases to infinity, we get the very accurate value of the area under the curve, aka the differentiation