18. The base of a solid is the circular region bounded by the graph of x^2+y^2 = a^2, where a > 0. Find the volume of the solid if every cross section perpendicular to the x-axis is a square. 19. Write the integral that gives the volume of a solid whose base is a circle centered at the origin, with diameter 6, if cross sections perpendicular to the x-axis are equilateral triangles