Some important issue of graph

Before moving on to the next example, there are some important things to note.

Firstly, in almost all problems a graph is pretty much needed.  Often the bounding region, that will give the limits of integration, is hard to determine without a graph.

Also, it can frequently be hard to determine which of the functions the upper function is and that is the lower function without any graph.

At last, If we obtain a negative number or zero we can be sure that we've committed a mistake somewhere and will have to go back and determine it.

Note that sometimes rather than saying region enclosed by we will say region bounded by. They mean the simialr thing.