How do you use the definition of the definite integral?
The definite integral can be used for a number of different purposes.
A common use is finding the area underneath the line on a graph; in this case, the definite integral is taken between the leftmost and rightmost points of the area in question.
Another use - and my personal favourite - is finding the volume, surface area, or similar attributes of solids of revolution. Often, one needs to use the definite integral twice, or three times, in one problem.