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Chapter 13. Multiple integrals. Section 13.2 Iterated integrals. Suppose f is a function of two variables that is integrable over the rectangle R = [a, b] × [c, d]. Rd We use notation f (x, y) dy to mean that x is held fixed and f (x, y) is integrated with respect c to y from y = c to y = d. This procedure is called partial integration with respect to y. Zd A(x) = f (x, y) dy c Zb a The integral Rb Rd a Zb Zd A(x) dx = f (x, y) dy dx c a f (x, y) dy dx is called an iterated integral. Thus, c Zb Zd a c Zb Zd f (x, y)dydx = f (x, y) dy dx a c means that we first integrate with respect to y from c to d and then with respect to x from a to b. Similarly, the iterated integral Zd Zb Zd Zb f (x, y)dxdy = f (x, y) dx dy c a c a means that we first integrate with respect to x from a to b and then with respect to y from c to d. Example 1. Evaluate the iterated integrals: 1. R3 R1 √ x + y dxdy 0 0 1 2. R1 R1 0 0 xy p dydx 2 x + y2 + 1 Fubini’s Theorem. If f is continuous on the rectangle R = [a, b] × [c, d], then Zd Zb ZZ f (x, y) dA = R Zb Zd f (x, y)dxdy = c a Example 2. Calculate the double integral ZZ f (x, y)dydx a xy 2 + R where R = {(x, y)|2 ≤ x ≤ 3, −1 ≤ y ≤ 0}. 2 y dA, x c Example 3. Find the volume of the solid lying under the elliptic paraboloid and above the rectangle R = [−1, 1] × [−2, 2]. 3 x2 y 2 + +z =1 4 9