The domain of the integral is often denoted by a single letter; for example: The domain does not necessarily have to be a constant. Functions can be used as the bounds as well. For example, or Multiple integrals can be solved by integrating for one variable at a time while treating the others as constants. For instance, to find the volume under the function in the region , we would integrate as follows: The order in which the variables are integrated does not matter. Another example: Area under in the region