How to Find the Limits of Integration for a Double Integral?

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Homework Help Overview

The discussion revolves around evaluating a double integral of the function (x+2y) over a region R in the first quadrant, which is bounded by the circle defined by the equation x²+y²=9. Participants are focused on determining the appropriate limits of integration for this double integral.

Discussion Character

  • Exploratory, Assumption checking, Problem interpretation

Approaches and Questions Raised

  • Participants discuss the limits for x and y, with some suggesting ranges based on the geometry of the circle. There is an exploration of how to describe the region R accurately, with caution against oversimplifying it to a square.

Discussion Status

The discussion is progressing with participants confirming the limits for y as 0 to sqrt(9-x²) and for x as 0 to 3. There is an acknowledgment of the need to determine the order of integration, and guidance has been provided regarding the setup of the iterated integral.

Contextual Notes

Participants are navigating the challenge of accurately defining the limits of integration without falling into the trap of misrepresenting the region as a square, which does not reflect the actual bounded area of the circle.

Math10
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Homework Statement


Evaluate the double integral (x+2y)dA, where R is the region in the first quadrant bounded by the circle x^2+y^2=9.

Homework Equations


None.

The Attempt at a Solution


I know how to evaluate the double integral but I just don't know how to find the limits of integration. I know that the radius of the circle is 3.
 
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First, what are the limits for x?
 
I have no idea. Is it 0 to 3?
 
It looks like you do have an idea. That's right.

For each x, what are the limits of y? You may need to draw the area.
 
From 0 to 3?
 
Math10 said:

Homework Statement


Evaluate the double integral (x+2y)dA, where R is the region in the first quadrant bounded by the circle x^2+y^2=9.

Homework Equations


None.

The Attempt at a Solution


I know how to evaluate the double integral but I just don't know how to find the limits of integration. I know that the radius of the circle is 3.
How would you describe the region R as a set? IOW, R = {(x, y) | <inequality involving x> and <inequality involving y>}.

If you are tempted to write the inequalities as ##0 \le x \le 3, \text{and } 0 \le y \le 3##, resist that temptation! That region would be a square, which the one in this problem is not.

As I said in another of your threads, many times carrying out the integration is the easy part, requiring very little thinking. Figuring out the limits of integration is often the more difficult task. Your textbook should have a number of examples. Have you looked at them?
 
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Math10 said:
From 0 to 3?
That would be a square in the first quadrant.
 
So y=sqrt(9-x^2) and y= - sqrt(9-x^2)?
 
Math10 said:
So y=sqrt(9-x^2) and y= - sqrt(9-x^2)?
Why would y be negative?
 
  • #10
So y=0 to sqrt(9-x^2)?
 
  • #11
Math10 said:
So y=0 to sqrt(9-x^2)?
Yes. Now what are your limits for x?
 
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  • #12
0 to 3?
 
  • #13
Yes. Now you need to figure the order in which you're going to integrate.
 
  • #14
dy dx?
 
  • #15
Math10 said:
dy dx?
You tell me ...
 
  • #16
dy dx.
 
  • #17
OK. Now you're ready to set up the iterated integral and evaluate it...
 
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  • #18
Thank you!
 

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