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Evaluating an improper double integral in polar coordinates

Evaluate the integral R 2 e −10 ( x 2 + y 2 ) d x d y .

This is an improper integral because we are integrating over an unbounded region R 2 . In polar coordinates, the entire plane R 2 can be seen as 0 θ 2 π , 0 r .

Using the changes of variables from rectangular coordinates to polar coordinates, we have

R 2 e −10 ( x 2 + y 2 ) d x d y = θ = 0 θ = 2 π r = 0 r = e −10 r 2 r d r d θ = θ = 0 θ = 2 π ( lim a r = 0 r = a e −10 r 2 r d r ) d θ = ( θ = 0 θ = 2 π d θ ) ( lim a r = 0 r = a e −10 r 2 r d r ) = 2 π ( lim a r = 0 r = a e −10 r 2 r d r ) = 2 π lim a ( 1 20 ) ( e −10 r 2 | 0 a ) = 2 π ( 1 20 ) lim a ( e −10 a 2 1 ) = π 10 .
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Evaluate the integral R 2 e −4 ( x 2 + y 2 ) d x d y .

π 4

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Key concepts

  • To apply a double integral to a situation with circular symmetry, it is often convenient to use a double integral in polar coordinates. We can apply these double integrals over a polar rectangular region or a general polar region, using an iterated integral similar to those used with rectangular double integrals.
  • The area d A in polar coordinates becomes r d r d θ .
  • Use x = r cos θ , y = r sin θ , and d A = r d r d θ to convert an integral in rectangular coordinates to an integral in polar coordinates.
  • Use r 2 = x 2 + y 2 and θ = tan −1 ( y x ) to convert an integral in polar coordinates to an integral in rectangular coordinates, if needed.
  • To find the volume in polar coordinates bounded above by a surface z = f ( r , θ ) over a region on the x y -plane, use a double integral in polar coordinates.

Key equations

  • Double integral over a polar rectangular region R
    R f ( r , θ ) d A = lim m , n i = 1 m j = 1 n f ( r i j * , θ i j * ) Δ A = lim m , n i = 1 m j = 1 n f ( r i j * , θ i j * ) r i j * Δ r Δ θ
  • Double integral over a general polar region
    D f ( r , θ ) r d r d θ = θ = α θ = β r = h 1 ( θ ) r = h 2 ( θ ) f ( r , θ ) r d r d θ

In the following exercises, express the region D in polar coordinates.

D is the region of the disk of radius 2 centered at the origin that lies in the first quadrant.

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D is the region between the circles of radius 4 and radius 5 centered at the origin that lies in the second quadrant.

D = { ( r , θ ) | 4 r 5 , π 2 θ π }

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D is the region bounded by the y -axis and x = 1 y 2 .

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D is the region bounded by the x -axis and y = 2 x 2 .

D = { ( r , θ ) | 0 r 2 , 0 θ π }

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D = { ( x , y ) | x 2 + y 2 4 x }

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D = { ( x , y ) | x 2 + y 2 4 y }

D = { ( r , θ ) | 0 r 4 sin θ , 0 θ π }

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In the following exercises, the graph of the polar rectangular region D is given. Express D in polar coordinates.

A sector of an annulus D is drawn between theta = pi/4 and theta = pi/2 with inner radius 3 and outer radius 5.

D = { ( r , θ ) | 3 r 5 , π 4 θ π 2 }

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 A sector of an annulus D is drawn between theta = 3 pi/4 and theta = 5 pi/4 with inner radius 3 and outer radius 5.

D = { ( r , θ ) | 3 r 5 , 3 π 4 θ 5 π 4 }

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In the following graph, the region D is situated below y = x and is bounded by x = 1 , x = 5 , and y = 0 .

A region D is given that is bounded by y = 0, x = 1, x = 5, and y = x, that is, a right triangle with a corner cut off.
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In the following graph, the region D is bounded by y = x and y = x 2 .

A region D is drawn between y = x and y = x squared, which looks like a deformed lens, with the bulbous part below the straight part.

D = { ( r , θ ) | 0 r tan θ sec θ , 0 θ π 4 }

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In the following exercises, evaluate the double integral R f ( x , y ) d A over the polar rectangular region D .

f ( x , y ) = x 2 + y 2 , D = { ( r , θ ) | 3 r 5 , 0 θ 2 π }

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f ( x , y ) = x + y , D = { ( r , θ ) | 3 r 5 , 0 θ 2 π }

0

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f ( x , y ) = x 2 + x y , D = { ( r , θ ) | 1 r 2 , π θ 2 π }

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f ( x , y ) = x 4 + y 4 , D = { ( r , θ ) | 1 r 2 , 3 π 2 θ 2 π }

63 π 16

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f ( x , y ) = x 2 + y 2 3 , where D = { ( r , θ ) | 0 r 1 , π 2 θ π } .

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f ( x , y ) = x 4 + 2 x 2 y 2 + y 4 , where D = { ( r , θ ) | 3 r 4 , π 3 θ 2 π 3 } .

3367 π 18

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f ( x , y ) = sin ( arctan y x ) , where D = { ( r , θ ) | 1 r 2 , π 6 θ π 3 }

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f ( x , y ) = arctan ( y x ) , where D = { ( r , θ ) | 2 r 3 , π 4 θ π 3 }

35 π 2 576

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D e x 2 + y 2 [ 1 + 2 arctan ( y x ) ] d A , D = { ( r , θ ) | 1 r 2 , π 6 θ π 3 }

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D ( e x 2 + y 2 + x 4 + 2 x 2 y 2 + y 4 ) arctan ( y x ) d A , D = { ( r , θ ) | 1 r 2 , π 4 θ π 3 }

7 576 π 2 ( 21 e + e 4 )

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In the following exercises, the integrals have been converted to polar coordinates. Verify that the identities are true and choose the easiest way to evaluate the integrals, in rectangular or polar coordinates.

Practice Key Terms 1

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Source:  OpenStax, Calculus volume 3. OpenStax CNX. Feb 05, 2016 Download for free at http://legacy.cnx.org/content/col11966/1.2
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