18:09 why the range of r goes from 0 to 1? I think the definition of S in this problem needs further clarification. S should be a paraboloid of z = x^2 + y^2 between z=0 and z=1. It is a little confusing by justing saying "above the unit disk".####[['S', 'Mathematical_Related_Concept', 'NEG'], ['r', 'Mathematical_Related_Concept', 'NEG']]