Iterated Integrals

Definition

Iterated integrals are a way to compute double integrals by breaking them down into a sequence of single integrals. This technique allows for the evaluation of integrals over two-dimensional regions by integrating with respect to one variable at a time, which can simplify calculations, especially in complex regions.

5 Must Know Facts For Your Next Test

1. Iterated integrals can be expressed as $$int_a^b int_c^d f(x, y) \, dy \, dx$$, where you first integrate with respect to y and then with respect to x.
2. The limits of integration for iterated integrals depend on the region of integration and must be determined carefully to ensure accurate results.
3. Changing the order of integration when using iterated integrals often requires adjusting the limits of integration according to Fubini's Theorem.
4. In cases where the region of integration is not rectangular, it may require breaking down into sub-regions to properly apply iterated integrals.
5. Iterated integrals are particularly useful in applications such as finding volumes under surfaces or calculating areas in non-rectangular regions.

Review Questions

• How do you set up an iterated integral for a given two-dimensional region, and what steps do you take to determine the limits of integration?
  • To set up an iterated integral for a given two-dimensional region, start by identifying the region's boundaries. Sketching the region can help visualize it. Once the boundaries are established, determine the outer integral's limits based on one variable while expressing the inner integral's limits as functions of that variable. This process ensures that both limits properly enclose the area of interest and allows for correct evaluation of the integral.
• Discuss how Fubini's Theorem applies to iterated integrals and what implications it has for changing the order of integration.
  • Fubini's Theorem allows you to change the order of integration in iterated integrals if certain conditions are met, typically involving continuity of the function being integrated. This flexibility means that if one order is complicated, switching to another order might simplify calculations. However, when doing so, it's crucial to re-evaluate and adjust the limits of integration accordingly to ensure that they still encompass the same region.
• Evaluate the iterated integral $$int_0^1 \fint_0^{x^2} (x + y) \, dy \, dx$$ and explain each step in your solution.
  • To evaluate the iterated integral $$\fint_0^1 \fint_0^{x^2} (x + y) \, dy \, dx$$, start by integrating with respect to y. The inner integral becomes $$\fint_0^{x^2} (x + y) \, dy = [xy + \frac{y^2}{2}]_0^{x^2} = x\cdot x^2 + \frac{(x^2)^2}{2} = x^3 + \frac{x^4}{2}$$. Next, substitute this result into the outer integral: $$\fint_0^1 (x^3 + \frac{x^4}{2}) \, dx = [\frac{x^4}{4} + \frac{x^5}{10}]_0^1 = \frac{1}{4} + \frac{1}{10} = \frac{5}{20} + \frac{2}{20} = \frac{7}{20}$$. Thus, the value of the iterated integral is $$\frac{7}{20}$$.

Related terms

Double Integral: A double integral is an integral that computes the accumulation of a function of two variables over a two-dimensional area, often expressed as $$int int f(x, y) \,dA$$.

Fubini's Theorem: Fubini's Theorem states that under certain conditions, the order of integration in a double integral can be interchanged without affecting the result.

Region of Integration: The region of integration is the specific area in the two-dimensional plane over which the function is being integrated, which can be bounded by curves or lines.