I have to calculate the volume of a sphere of radius 2 that has a hole with radius 1 through the sphere and that includes the center of the sphere. I am trying to solve this by putting semi disk with lenght 4 units and 1 unit from the base to the top, and then revolving this disk around the line y=1. Then I will get a sphere of radius 2 with a hole through it with radius 1. I don't know if this is a good approach but I have a problem with the equation of the semi circle. I don´t know how to equate this semicircle, since y=sqrt(a^2-x^2) is not working. Could someone please give me a hint?