## Calculus: Early Transcendentals (2nd Edition)

The net area bounded by the graph of the function and $x$ axis on the interval of integration.
The definite integral for the pieces of the graph of the function that are above $x$ axis gives their area while for those that are under the $x$ axis gives minus their area so in total, if the function changes its sign in the interval of integration, all the areas above $x$ axis are added and all the areas below $x$ axis are subtracted so we get the net area on the interval of integration.