Change of variable in a double integral

In summary, the conversation discusses the process of solving a question involving an ellipse and the use of the Jacobian. The resulting figure is a circle of radius 1, centered at the origin. The range for r is from 0 to 1 and the range for theta is from 0 to 2pi in order to sweep out the whole circle.
  • #1
rashida564
220
6
Homework Statement
The image bellow is the question
Relevant Equations
Jacobian
Hi everyone, I tried to solve the last part of the question, I substituted back the expression of x and y into the equation of the ellipse, I got that r=1 or r=-1. But got no idea how to find the boundary for theta, I got a guess that, It should be from zero to pi. But got no reason why to believe this.
 

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  • #2
rashida564 said:
Homework Statement:: The image bellow is the question
Homework Equations:: Jacobian

Hi everyone, I tried to solve the last part of the question, I substituted back the expression of x and y into the equation of the ellipse, I got that r=1 or r=-1. But got no idea how to find the boundary for theta, I got a guess that, It should be from zero to pi. But got no reason why to believe this.
After the transformation, the resulting figure is a circle of radius 1, centered at the origin. If r ranges from 0 to 1, what needs to be the range for ##\theta## to sweep out the whole circle?
 
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Likes vanhees71 and scottdave
  • #3
From zero to two pi, thanks man
 
Last edited by a moderator:
  • #4
rashida564 said:
From zero to two pi, thanks man
Right.

BTW, I edited your post. You quoted me as saying "From zero to two pi." I didn't say this, so it's misleading to quote me.
 

1. What is a change of variable in a double integral?

A change of variable in a double integral is a technique used to simplify the integration of a function over a region in two variables. It involves substituting a new set of variables in place of the original variables, which allows for a more manageable integral to be evaluated.

2. Why is a change of variable useful in double integrals?

A change of variable can be useful in double integrals because it can help to simplify the integrand and make it easier to evaluate the integral. It can also allow for the use of different coordinate systems, which may be more suitable for certain types of functions or regions.

3. How do you choose the appropriate variables for a change of variable in a double integral?

The choice of variables for a change of variable in a double integral depends on the shape of the region being integrated over and the function being integrated. Generally, the goal is to find a substitution that will transform the region into a simpler shape, such as a rectangle or a circle, and make the integrand easier to work with.

4. What are the steps for performing a change of variable in a double integral?

The steps for performing a change of variable in a double integral are as follows: 1) Determine the new variables to be used and the transformation equations that relate them to the original variables. 2) Express the original integral in terms of the new variables. 3) Determine the limits of integration for the new variables by transforming the limits for the original variables. 4) Solve the transformed integral using the new limits and the new variables.

5. Can a change of variable be applied to any double integral?

In theory, a change of variable can be applied to any double integral. However, in practice, it may not always lead to a simpler integral or be possible to perform. It is important to carefully consider the region and function involved before attempting to use a change of variable.

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