Double integral over a region needing polar coordinates.

In summary, the given double integral is evaluated by converting to polar coordinates over the region R = {(x,y) | 1≤x^2+y^2≤4, 0≤y≤x}. By using the polar coordinates x=rcosθ and y=rsinθ, the integral becomes ∫∫arctan(tan(θ))r drdθ. The region can be visualized as two circles with radii 1 and 2, separated by 0≤y≤x. The bounds for θ are 0 to π/4, representing the half of the region under the diagonal x=y.
  • #1
Tropicalism
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1. Evaluate the double integral ∫∫arctan(y/x) dA by converting to polar coordinates over the Region R= { (x,y) | 1≤x^2+y^2≤4 , 0≤y≤x }


My attempt at solving

Converting to polar using x=rcosθ and y=rsinθ I get
∫∫arctan(tan(θ))r drdθ


I understand that I have to integrate first with respect to r and then with respect to θ but I'm not sure what the region is supposed to look like over which I am integrating so I don't know what my bounds should be. I understand that 1≤x^2+y^2≤4 is basically 2 circles, one with radius 1, and one with radius 2 and my region is between them. I don't know how 0≤y≤x affects it though, I thought it would just make it the top right portion of the circle but that's incorrect.
Any help is greatly appreciated, thanks.
 
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  • #2
Hi Tropicalism! Welcome to PF! :smile:
Tropicalism said:
I don't know how 0≤y≤x affects it though, I thought it would just make it the top right portion of the circle but that's incorrect.

"0≤y and 0≤x" would be top right portion (the first quadrant :wink:)

"0≤y≤x" is the half of it under the diagonal x=y :smile:
 
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  • #3
tiny-tim said:
Hi Tropicalism! Welcome to PF! :smile:


"0≤y and 0≤x" would be top right portion (the first quadrant :wink:)

"0≤y≤x" is the half of it under the diagonal x=y :smile:
Oh, ok thank you. So using polar that portion would end at θ=∏/4 correct?
 
  • #4
yup! :biggrin:
 
  • #5
tiny-tim said:
yup! :biggrin:

Thanks! Thats all I needed.
 

Related to Double integral over a region needing polar coordinates.

What is a double integral over a region needing polar coordinates?

A double integral over a region needing polar coordinates is a mathematical concept used to find the volume or area of a region in the Cartesian coordinate system by converting the coordinates to polar coordinates.

When is it necessary to use polar coordinates in a double integral?

Polar coordinates are necessary in a double integral when the region being integrated is better described in terms of radial distance and angle from a central point, rather than its x and y coordinates.

How do you convert a double integral from Cartesian to polar coordinates?

To convert a double integral from Cartesian to polar coordinates, you need to substitute the x and y variables with their corresponding polar coordinate equivalents, and also change the bounds of integration to reflect the new coordinate system.

What is the formula for calculating a double integral over a region needing polar coordinates?

The formula for calculating a double integral over a region needing polar coordinates is ∬f(r,θ)rdrdθ, where f(r,θ) is the function being integrated, r is the radial distance, and θ is the angle.

What are some real-world applications of double integrals over regions needing polar coordinates?

Double integrals over regions needing polar coordinates are commonly used in physics and engineering to calculate the mass, center of mass, and moments of inertia of objects with circular or symmetrical shapes. They are also used in calculating electric and gravitational fields in polar coordinate systems.

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