Double integral problem (pretty basic)

In summary, the speaker is struggling with double/triple integrals in their multivariable calculus course and is seeking help for a specific problem involving a fraction with x and y as variables. They have attempted u-substitution and polynomial long division but are still struggling and considering dropping the course. Another person suggests trying to integrate with respect to y first, which leads the speaker to solve the problem successfully.
  • #1
chexmix
8
0
Good day, all:

We recently hit double/triple integrals in my multivariable calculus course and I have found that my integration abilities are, well, *beyond* rusty ... and so the problem below, which is one of the very first on my current problem set, has me stumped.

Homework Statement



[itex]\int\int_{R}\frac{x}{1+xy} dA[/itex] R = { [0,1]x[0,1] = {(x,y): 0 [itex]\leq[/itex] x [itex]\leq[/itex]1, 0 [itex]\leq[/itex] y [itex]\leq[/itex]1 }

The Attempt at a Solution



My first and nearly only step was to turn this into an iterated integral:

[itex]\int^{1}_{0}\int^{1}_{0}\frac{x}{1+xy} dx dy[/itex]

... and this is where I begin to choke and sputter. I have started to try u-substitution on this with u = 1+xy, but didn't get anything that made sense to me; some hints I have found online seem to indicate that I should be able to perform "polynomial long division" to turn this into a sum or difference of two simpler integrals, but I guess I don't sufficiently understand polynomial long division to carry this out here.

Any hints would be much appreciated. I am considering dropping this course, but I would like to avoid that ... I need some serious integration mojo to be directly infused into my skull asap. :bugeye:

Regards,

Glenn
 
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  • #2
Did you try integrating with respect to y first?
 
  • #3
murmillo said:
Did you try integrating with respect to y first?

No. I'll check it out and see what happens. Thank you.

EDIT: That was key, of course. Jeez. ::strikes forehead::
 
Last edited:

Related to Double integral problem (pretty basic)

1. What is a double integral?

A double integral is a mathematical concept used in calculus to find the signed area between a given function and the x-y plane. It involves integrating a function over a two-dimensional region, typically represented by a rectangle on a graph.

2. What is the purpose of a double integral?

The purpose of a double integral is to find the volume under a surface in three-dimensional space. It can also be used to find the area of a curved surface or the mass of an object with varying density.

3. How do you solve a double integral?

To solve a double integral, you first need to determine the limits of integration for both the x and y variables, based on the given function and the region of integration. Then, you can use various integration techniques such as the rectangular or polar methods to evaluate the integral and find the final answer.

4. What is the difference between a double integral and a single integral?

A single integral is used to find the area under a curve in one dimension, while a double integral is used to find the volume under a surface in two dimensions. A single integral has one variable of integration, while a double integral has two variables of integration.

5. In what real-life situations is a double integral used?

Double integrals have many practical applications in fields such as physics, engineering, and economics. They can be used to calculate the work done by a force, the center of mass of an object, or the average value of a function. They are also used in probability and statistics to find the joint probability of two events.

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