Remark 9.2.6 These were perfectly made to describe slices. Assuming Fubini's theorem holds, you can express integrals over x-simple sets S as an iterated double integral dydx. J, paA = [."S 4(x, y)dydx Similarly, you can express integrals over y-simple sets T as an iterated double integral dxdy. p(x, y)dxdy.Definition 9.2.5 A set S C R2 is x-simple if there exist continuous f : [a, b] - R and g : [a, b] - R such that S = {(x, y)ER2 : agx