Double integrals: cartesian to polar coordinates

In summary, the conversation discusses changing a Cartesian integral into a polar integral and finding the bounds for both r and θ. The area of integration forms a right triangle with a constant projection of r*sinθ, and the limit for r is a function of θ. The final answer for the problem is 36.
  • #1
mmont012
39
0

Homework Statement


Change the Cartesian integral into an equivalent polar integral and then evaluate.

Homework Equations


x=rcosθ
y=rsinθ

upload_2015-11-27_1-53-51.png

I have:
∫∫r2cosθ dr dθ

The bounds for theta would be from π/4 to π/2, but what would the bounds for r be?

I only need help figuring out the bounds, not with the evaluating.

The answer for the problem is 36 (or so says the back of the textbook).
 
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  • #2
mmont012 said:
but what would the bounds for r be?
The area of integration forms a right triangle subtending angle from 45 to 90 degree, so the limit for r would be a function of ##\theta##. For a hint, as you sweep the triangle in between those two limiting angles, the projection ##r \sin \theta## is constant.
 
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  • #3
For every theta, a line from the origin, making angle theta with the y-axis, to the line y= 6 is the hypotenuse of a right triangle with one leg of length 6. [itex]cos(\theta)= \frac{6}{h}[/itex].
 
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  • #4
Thank you both!
 

1. What is the difference between Cartesian and polar coordinates?

Cartesian coordinates are a system of specifying points in a plane using x and y coordinates. Polar coordinates, on the other hand, use a distance (r) and angle (θ) to specify a point in a plane.

2. How do you convert from Cartesian to polar coordinates?

To convert from Cartesian to polar coordinates, you can use the following formulas:
r = √(x²+y²) and θ = tan^-1 (y/x).
Alternatively, you can use the Pythagorean theorem and trigonometric identities to calculate the values.

3. What is a double integral in polar coordinates?

A double integral in polar coordinates is an integral with two variables, r and θ, where the limits of integration are determined by the region of integration in the polar plane. It is used to calculate the volume or area of a region in polar coordinates.

4. How do you evaluate a double integral in polar coordinates?

To evaluate a double integral in polar coordinates, you first need to determine the limits of integration for r and θ based on the region of integration. Then, you can use the appropriate formula for the function being integrated and solve the integral using techniques such as substitution or integration by parts.

5. What are some applications of double integrals in polar coordinates?

Double integrals in polar coordinates have many applications in mathematics and physics. They are commonly used in calculating the area or volume of irregular shapes, finding the center of mass and moments of inertia, and solving differential equations in polar coordinates. They also have applications in fields such as engineering, economics, and computer graphics.

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