Let f (x) be defined and continuous in a real interval a, b. The definite integral of f (x), of The up until B, is a real number, and is indicated by the symbol:
Where:
The is the lower limit of integration;
B is the upper limit of integration;
f (x) It is the integrand.
If represents the area between the axis x and the curve f (x), for

If represents the area between the curves so