Riemann sum  Integrals
Riemann sum
In this lesson, we will learn how to approximate the area under the curve using rectangles. First we notice that finding the area under the curve is easy if the function is a straight line. However, what if it is a curve? We will actually have to approximate curves using a method called "Riemann Sum". This method involves finding the length of each subinterval (delta x), and finding the points of interest, finding the y values of each point of interest, and then use the find the area of each rectangle to sum them up. There are 3 methods in using the Riemann Sum. First is the "Right Riemann Sum", second is the "Left Riemann Sum", and third is the "Middle Riemann Sum". Lastly, we will look at the idea of infinite subintervals (which leads to integrals) to exactly calculate the area under the curve.
Basic concepts:
 Riemann sum
Related concepts:
 Areas between curves
 Average value of a function
Lessons
Notes:

Intro Lesson
Riemann Sum overview:

2.
Finding the area under the graph of a function using the Riemann Sum.
Consider the function $f\left( x \right) = {x^2}$, $1 \le x \le 3$.
Estimate the area under the graph of $f$ using four approximating rectangles and taking the sample points to be: 
3.
Evaluating integrals with a Riemann Sum
Consider the function $f\left( x \right) = {x^2}  5x + 3$, $2 \le x \le 5$.