Iterated Integrals and Fubini's Theorem

An iterated integral expresses a double integral as two successive single integrals evaluated one after another. For a function over a rectangular region, you first integrate with respect to one variable while holding the other constant, then integrate the resulting function with respect to the second variable. This process reduces a complex two-variable problem into manageable one-variable integrations.

Holding one variable constant creates a cross-sectional slice through the region. For example, fixing x and integrating with respect to y produces a function representing the area of a vertical slice at that x-position. Integrating these cross-sectional areas along the x-axis yields the total volume under the surface, transforming the double integral into a manageable sequence of single integrals.

Fubini's Theorem states that for a continuous function on a rectangular region, both orders of integration produce identical results. You can integrate first with respect to y then x, or first with respect to x then y, and obtain the same answer. This flexibility allows you to choose whichever order makes the calculation simpler or more convenient.

For a thin sheet with variable density, integrate the density function with respect to one variable to find the mass of a thin strip. Then integrate these strip masses with respect to the other variable to obtain total mass. Fubini's Theorem ensures that integrating density first along one direction then the other yields the same total mass, enabling flexible problem-solving approaches.

Fubini's Theorem guarantees that for continuous functions on rectangular regions, both integration orders yield the same result. This flexibility arises because the double integral represents a well-defined geometric quantity—volume or mass—independent of the computational path. Students can select the order that simplifies calculations, such as choosing changing the order of integration in double integrals when one sequence produces easier antiderivatives.

Cross-sectional area represents the area of a vertical or horizontal slice through a solid at a fixed position. By integrating this area function across the entire region, you accumulate all cross-sections to obtain total volume. This geometric interpretation makes iterated integration intuitive: each inner integral computes a cross-sectional area, and the outer integral sums these areas to yield volume.

For rectangular regions defined by fixed bounds a ≤ x ≤ b and c ≤ y ≤ d, Fubini's Theorem ensures that the double integral equals either iterated integral: integrating first with respect to y from c to d, then x from a to b, or vice versa. The constant bounds and continuity of the function guarantee both orders produce identical results, simplifying evaluation of volumes and other quantities.