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Chapter 15 – Multiple Integrals 15.4 Double Integrals in Polar Coordinates 1 Objectives: Determine how to express double integrals in polar coordinates Dr. Erickson
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Polar Coordinates Review Recall from this figure that the polar coordinates (r, θ) of a point are related to the rectangular coordinates (x, y) by the equations ◦ r 2 = x 2 + y 2 ◦ x = r cos θ ◦ y = r sin θ 15.4 Double Integrals in Polar Coordinates2Dr. Erickson
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Double Integrals in Polar Coordinates Suppose that we want to evaluate a double integral, where R is one of the regions shown here. 15.4 Double Integrals in Polar Coordinates3Dr. Erickson
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Double Integrals in Polar Coordinates In either case, the description of R in terms of rectangular coordinates is rather complicated but R is easily described by polar coordinates. 15.4 Double Integrals in Polar Coordinates4Dr. Erickson
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Polar Rectangle The regions in the first figure are special cases of a polar rectangle R = {(r, ) | a ≤ r ≤ b, ≤ ≤ } shown here. 15.4 Double Integrals in Polar Coordinates5Dr. Erickson
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Polar Rectangle To compute the double integral where R is a polar rectangle, we divide: ◦ The interval [a, b] into m subintervals [r i–1, r i ] of equal width ∆r = (b – a)/m. ◦ The interval [ , ] into n subintervals [ j–1, j ] of equal width ∆ = ( – )/n. 15.4 Double Integrals in Polar Coordinates6Dr. Erickson
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Polar Subrectangle The “center” of the polar subrectangle R ij = {(r, θ) | r i–1 ≤ r ≤ r i, θ j–1 ≤ θ ≤ θ i } has polar coordinates r i * = ½ (r i–1 + r i ) θ j * = ½ (θ j–1 + θ j ) 15.4 Double Integrals in Polar Coordinates7Dr. Erickson
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Polar Rectangles The rectangular coordinates of the center of R ij are (r i * cos θ j *, r i * sin θ j *). So, a typical Riemann sum is shown with Equation 1: 15.4 Double Integrals in Polar Coordinates8Dr. Erickson
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Polar Rectangles If we write g(r, θ) = r f(r cos θ, r sin θ), the Riemann sum in Equation 1 can be written as: ◦ This is a Riemann sum for the double integral 15.4 Double Integrals in Polar Coordinates9Dr. Erickson
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Polar Rectangles Thus, we have: 15.4 Double Integrals in Polar Coordinates10Dr. Erickson
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Change to Polar Coordinates 15.4 Double Integrals in Polar Coordinates11Dr. Erickson
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Change to Polar Coordinates Formula 2 says that we convert from rectangular to polar coordinates in a double integral by: ◦ Writing x = r cos θ and y = r sin θ ◦ Using the appropriate limits of integration for r and θ ◦ Replacing dA by dr dθ Be careful not to forget the additional factor r on the right side of Formula 2. 15.4 Double Integrals in Polar Coordinates12Dr. Erickson
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Change to Polar Coordinates A classical method for remembering the formula is shown here. ◦ The “infinitesimal” polar rectangle can be thought of as an ordinary rectangle with dimensions r dθ and dr. ◦ So, it has “area” dA = r dr dθ. 15.4 Double Integrals in Polar Coordinates13Dr. Erickson
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Example 1 Sketch the region whose area is given by the integral and evaluate the integral. 15.4 Double Integrals in Polar Coordinates14Dr. Erickson
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Example 2 Evaluate the integral by changing to polar coordinates. 15.4 Double Integrals in Polar Coordinates15Dr. Erickson
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More Complicated Regions What we have done so far can be extended to the more complicated type of region shown here. ◦ It’s similar to the type II rectangular regions considered in Section 15.3 15.4 Double Integrals in Polar Coordinates16Dr. Erickson
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More Complicated Volumes If f is continuous on a polar region of the form D = {(r, θ) | α ≤ θ ≤ β, h 1 (θ) ≤ r ≤ h 2 (θ)} then 15.4 Double Integrals in Polar Coordinates17Dr. Erickson
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Volumes and Areas In particular, taking f(x, y) = 1, h 1 (θ) = 0, and h 2 (θ) = h(θ) in the formula, we see that the area of the region D bounded by θ = α, θ = β, and r = h(θ) is: Which agrees with formula 3 in section 10.4. 15.4 Double Integrals in Polar Coordinates18Dr. Erickson
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Example 3 Use a double integral to find the area of the region. 15.4 Double Integrals in Polar Coordinates19Dr. Erickson
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Example 4 Use polar coordinates to find the volume of the given solid. 15.4 Double Integrals in Polar Coordinates20Dr. Erickson
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Example 5 Evaluate the iterated integral by converting to polar coordinates. 15.4 Double Integrals in Polar Coordinates21Dr. Erickson
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More Examples The video examples below are from section 14.6 in your textbook. Please watch them on your own time for extra instruction. Each video is about 2 minutes in length. ◦ Example 2 Example 2 ◦ Example 3 Example 3 ◦ Example 4 Example 4 15.4 Double Integrals in Polar Coordinates22Dr. Erickson
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Demonstrations Feel free to explore these demonstrations below. ◦ Double Integral for Volume Double Integral for Volume 15.4 Double Integrals in Polar Coordinates23Dr. Erickson
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