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15. Multiple Integrals. 15.1. Double Integrals over Rectangles. Volumes and Double Integrals. In 3D, a volume can be written as a double integral:. Properties of Double Integrals. Linearity properties:

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## Multiple Integrals

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**15**• Multiple Integrals**15.1**• Double Integrals over Rectangles**Volumes and Double Integrals**• In 3D, a volume can be written as a double integral:**Properties of Double Integrals**• Linearity properties: • [f (x, y) + g(x, y)] dA = f (x, y) dA + g(x, y) dA • c f (x, y) dA = c f (x, y) dA where c is a constant • Inequality property: • If f (x, y) g(x, y) for all (x, y) in R, then • f (x, y) dA g(x, y) dA**Example:**• Evaluate: • Answer:**Average Values**• Recall that the average value of a function f of one variable defined on an interval [a, b] is: • In a similar fashion we define the average value of a function f of two variables on a rectangle R to be: • where A(R) is the area of R.**Example**• Evaluate the average value of the function f(x,y)= • over the rectangle: x: [0,4] y: [0,1] • Answer:**15.2**• Iterated integrals**Iterated Integrals**• Suppose that f is a function of two variables that is integrable on the rectangle R = [a, b] [c, d ]. • We use the notation to mean that x is held fixed and • f (x, y) is integrated with respect to y from y = c to y = d. This procedure is called partial integration with respect to y. (Notice its similarity to partial differentiation.) • Now is a number that depends on the value of x, so it defines a function of x:**Iterated Integrals**• If we now integrate the function A with respect to x from x = a to x = b, we get • The integral on the right side of Equation 1 is called an iterated integral. Usually the brackets are omitted. • means that we first integrate with respect to y from c to d and then with respect to x from a to b.**Iterated Integrals**• Similarly, the iterated integral • means that we first integrate with respect to x (holding y fixed) from • x = a to x = b and then we integrate the resulting function of y with respect to y from y = c to y = d. • Notice that in both Equations 2 and 3 we work from the inside out.**Example 1**• Evaluate the iterated integrals. • (b) • Solution: • Regarding x as a constant, we obtain**Example 1 – Solution**• cont’d • Thus the function A in the preceding discussion is given by in this example. • We now integrate this function of x from 0 to 3:**Example 1 – Solution**• cont’d • (b) Here we first integrate with respect to x:**Iterated Integrals**• Notice that in Example 1 we obtained the same answer whether we integrated with respect to y or x first. • In general, it turns out (by Theorem) that the two iterated integrals in Equations 2 and 3 will be equal; that is, the order of integration does not matter. • (This is similar to Clairaut’s Theorem on the equality of the mixed partial derivatives.)**Fubini’s Theorem**• Theorem:**Iterated Integrals - special case:**• In the special case where f (x, y) can be factored as the product of a function of x only and a function of y only, the double integral of f can be written in a particularly simple form. • To be specific, suppose that f (x, y) = g(x)h(y) and R = [a, b] [c, d]. • Then Fubini’s Theorem will lead to:

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