The general sum \(\displaystyle{ \sum_{i=1}^{n}{[f(x_i) \cdot \Delta x_i]} }\) is an approximation to the area under the curve \(f(x)\). Let's look more closely at this sum and get an idea of what is going on.

For each \(i\), we calculate the area of a rectangle on the interval \([a,b]\). The height of the rectangle is \(f(x_i)\) and the width is \(\Delta x_i\). If the same area is broken into more and more rectangles, we can get a better and better approximation of the area.

To get the actual area, we can take the limit of the above sum as follows.
\(\displaystyle{ \lim_{n \to \infty}{ \left[ \sum_{i=1}^{n}{(f(x_i) \cdot \Delta x_i)} \right] } = \int_{a}^{b}{f(x)~dx} }\)
Let's break down the equation. When taking the limit we get

\(\displaystyle{ \lim_{n \to \infty}\sum_{i=1}^{n}{} ~~~ \to ~~~ \int_{a}^{b}{} }\)

\(\Delta x_i ~~~ \to ~~~ dx \)

The idea is that the width of each interval goes to \(dx\). The limit of the sum goes to \(\int_{a}^{b}{}\). You can think of the integrand, \(f(x)\), being swept from \(a\) to \(b\).

Okay, time for a video. Here is a good video on the definition of the definite integral. It will give you an intuitive understanding what it means and how the notation works.

Before we actually evaluate the Reimann sum with limits, let's see how to approximate the value of a definite integral. This is a great video with explanation of an example.

From the first video in this section, you know that the limit given above is the definition of the definite integral using Reimann Sums. Here is an example showing how to calculate a definite integral using this definition. The example is worked in two consecutive videos.

Fortunately, we are not going to have to calculate these sums and limits all the time. The next page discusses the (First) Fundamental Theorem of Calculus, which we use extensively in calculus.