This document provides an explanation of how to calculate the volume of a solid with a cross-sectional area that changes along one axis. It gives the example of finding the volume of a solid where the cross-section is a triangle perpendicular to the x-axis, with base that varies as a function of x from 0 to 4. The document provides the formula for the area of the triangle as a function of x, and the integral required to calculate the volume by summing the areas of each cross-sectional slice from x=0 to x=4.